gpio: change inline functions to static inline

When compiling with optimization level 0 (-O0), the linker complains
about missing references to uart functions. This is due to compiler
treating this functions as extern, since -O0 disables function inlining,
as described in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49653

So change the declaration to static inline.

Change-Id: Ifd835bf0e667afb1cbf2e76bb490664af87c382d
Signed-off-by: Daniel Leung <daniel.leung@intel.com>
diff --git a/include/gpio.h b/include/gpio.h
index b7d94e1..5621735 100644
--- a/include/gpio.h
+++ b/include/gpio.h
@@ -97,8 +97,8 @@
  *  @param pin Pin number operate on.
  *  @param flags Flags for pin configuration. IN/OUT, interrupt ...
  */
-inline int gpio_pin_configure(struct device *port, uint8_t pin,
-			      int flags)
+static inline int gpio_pin_configure(struct device *port, uint8_t pin,
+				     int flags)
 {
 	struct gpio_driver_api *api;
 
@@ -112,7 +112,8 @@
  *  @param pin Pin number operate on.
  *  @param value Value to set the pin to.
  */
-inline int gpio_pin_write(struct device *port, uint32_t pin, uint32_t value)
+static inline int gpio_pin_write(struct device *port, uint32_t pin,
+				 uint32_t value)
 {
 	struct gpio_driver_api *api;
 
@@ -127,7 +128,8 @@
  *  @param pin Pin number operate on.
  *  @param value Integer pointer to receive the output of the read.
  */
-inline int gpio_pin_read(struct device *port, uint32_t pin, uint32_t *value)
+static inline int gpio_pin_read(struct device *port, uint32_t pin,
+				uint32_t *value)
 {
 	struct gpio_driver_api *api;
 
@@ -141,8 +143,8 @@
  *  @param port Pointer to device structure for driver instance.
  *  @param callback Application callback function.
  */
-inline int gpio_set_callback(struct device *port,
-			     gpio_callback_t callback)
+static inline int gpio_set_callback(struct device *port,
+				    gpio_callback_t callback)
 {
 	struct gpio_driver_api *api;
 
@@ -155,7 +157,7 @@
  *  @param port Pointer to device structure for driver instance.
  *  @param pin Pin number operate on.
  */
-inline int gpio_pin_enable_callback(struct device *port, uint32_t pin)
+static inline int gpio_pin_enable_callback(struct device *port, uint32_t pin)
 {
 	struct gpio_driver_api *api;
 
@@ -169,7 +171,7 @@
  *  @param port Pointer to device structure for driver instance.
  *  @param pin Pin number operate on.
  */
-inline int gpio_pin_disable_callback(struct device *port,  uint32_t pin)
+static inline int gpio_pin_disable_callback(struct device *port, uint32_t pin)
 {
 	struct gpio_driver_api *api;
 
@@ -183,7 +185,7 @@
  *  @param port Pointer to device structure for driver instance.
  *  @param flags Flags for port configuration. IN/OUT, interrupt ...
  */
-inline int gpio_port_configure(struct device *port, int flags)
+static inline int gpio_port_configure(struct device *port, int flags)
 {
 	struct gpio_driver_api *api;
 
@@ -196,7 +198,7 @@
  *  @param port Pointer to device structure for driver instance.
  *  @param value Value to set the pin to.
  */
-inline int gpio_port_write(struct device *port, uint32_t value)
+static inline int gpio_port_write(struct device *port, uint32_t value)
 {
 	struct gpio_driver_api *api;
 
@@ -210,7 +212,7 @@
  *  @param port Pointer to device structure for driver instance.
  *  @param value Integer pointer to receive the output of the read.
  */
-inline int gpio_port_read(struct device *port, uint32_t *value)
+static inline int gpio_port_read(struct device *port, uint32_t *value)
 {
 	struct gpio_driver_api *api;
 
@@ -223,7 +225,7 @@
  *  @brief Enable port callback.
  *  @param port Pointer to device structure for driver instance.
  */
-inline int gpio_port_enable_callback(struct device *port)
+static inline int gpio_port_enable_callback(struct device *port)
 {
 	struct gpio_driver_api *api;
 
@@ -236,7 +238,7 @@
  *  @brief Disable port callback.
  *  @param port Pointer to device structure for driver instance.
  */
-inline int gpio_port_disable_callback(struct device *port)
+static inline int gpio_port_disable_callback(struct device *port)
 {
 	struct gpio_driver_api *api;
 
@@ -248,7 +250,7 @@
  *  @brief Save the state of the device and go to low power state
  *  @param port Pointer to device structure for driver instance.
  */
-inline int gpio_suspend(struct device *port)
+static inline int gpio_suspend(struct device *port)
 {
 	struct gpio_driver_api *api;
 
@@ -260,7 +262,7 @@
  *  @brief Restore state stored during suspend and resume operation.
  *  @param port Pointer to device structure for driver instance.
  */
-inline int gpio_resume(struct device *port)
+static inline int gpio_resume(struct device *port)
 {
 	struct gpio_driver_api *api;