include/system_timer.h: Timer API cleanup

Rename timer driver API functions to be consistent.  ADD DOCS TO THE
HEADER so implementations understand what the requirements are.
Remove some unused functions that don't need declarations here.

Also removes the per-platform #if's around the power control callback
in favor of a weak-linked noop function in the driver initialization
(adds a few bytes of code to default platforms -- we'll live, I
think).

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
diff --git a/drivers/timer/nrf_rtc_timer.c b/drivers/timer/nrf_rtc_timer.c
index b5e812d..be9993e 100644
--- a/drivers/timer/nrf_rtc_timer.c
+++ b/drivers/timer/nrf_rtc_timer.c
@@ -93,10 +93,10 @@
  *
  * This function is not reentrant. It is called from:
  *
- * * _timer_idle_exit(), which in turn is called with interrupts disabled when
+ * * z_clock_idle_exit(), which in turn is called with interrupts disabled when
  * an interrupt fires.
  * * rtc1_nrf5_isr(), which runs with interrupts enabled but at that time the
- * device cannot be idle and hence _timer_idle_exit() cannot be called.
+ * device cannot be idle and hence z_clock_idle_exit() cannot be called.
  *
  * Since this function can be preempted, we need to take some provisions to
  * announce all expected sys ticks that have passed.
@@ -223,7 +223,7 @@
 {
 	u32_t max_cycles = _get_max_clock_time();
 
-	z_tick_set(_get_elapsed_clock_time());
+	z_tick_set(z_clock_uptime());
 	/* Update rtc_past to track rtc timer count*/
 	rtc_past = (z_tick_get() *
 			sys_clock_hw_cycles_per_tick()) & RTC_MASK;
@@ -309,7 +309,7 @@
 
 	/* Update expected_sys_ticls to time to programe*/
 	expected_sys_ticks = time;
-	z_tick_set(_get_elapsed_clock_time());
+	z_tick_set(z_clock_uptime());
 	/* Update rtc_past to track rtc timer count*/
 	rtc_past = (z_tick_get() * sys_clock_hw_cycles_per_tick()) & RTC_MASK;
 
@@ -367,7 +367,7 @@
  * returns : total number of sys ticks passed since device bootup.
  */
 
-u64_t _get_elapsed_clock_time(void)
+u64_t z_clock_uptime(void)
 {
 	u64_t elapsed;
 	u32_t rtc_elapsed, rtc_past_copy;
@@ -418,7 +418,7 @@
  * b) Schedule next sys tick at 400.
  *
  */
-void _timer_idle_exit(void)
+void z_clock_idle_exit(void)
 {
 #ifdef CONFIG_TICKLESS_KERNEL
 	if (!expected_sys_ticks && _sys_clock_always_on) {
@@ -509,7 +509,7 @@
 
 }
 
-int _sys_clock_driver_init(struct device *device)
+int z_clock_driver_init(struct device *device)
 {
 	struct device *clock;