drivers/gpio: stm32: Use gpio device as gpio_stm32_configure arg

Now that pinmux driver holds a table of GPIO device pointers,
use gpio device as the single source of trust for gpio_base
and remove use of port_base and related code.
This way, gpio_stm32_configure could directly take gpio device
pointer as argument.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>