FROMPULL: drivers: gpio: stm32: Fix compiler warning

The local function ll_gpio_get_pin_pull() is not always referenced.  Add
the __maybe_unused attribute to avoid compiler warnings (which may be
promoted to errors by downstream users).

Manaul cherry-pick of:
zephyrproject-rtos/zephyr#78416

Signed-off-by: Keith Short <keithshort@google.com>
(cherry picked from commit c2fba5ab421039a95fbc279af27415695e6af97c)

BUG=b:320527549
BUG=b:366515240
TEST=zmake build bloonchipper

Change-Id: Ia0c5dd5e7615e792ed43cc0e46d0598b0447ee93
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5861473
Tested-by: Keith Short <keithshort@chromium.org>
Reviewed-by: Tristan Honscheid <honscheid@google.com>
Commit-Queue: Keith Short <keithshort@chromium.org>

drivers/gpio/gpio_stm32.c

@@ -179,7 +179,7 @@ static inline void ll_gpio_set_pin_pull(GPIO_TypeDef *GPIOx, uint32_t Pin, uint3
 #endif /* CONFIG_SOC_SERIES_STM32WB0X */
 }
 
-static inline uint32_t ll_gpio_get_pin_pull(GPIO_TypeDef *GPIOx, uint32_t Pin)
+__maybe_unused static inline uint32_t ll_gpio_get_pin_pull(GPIO_TypeDef *GPIOx, uint32_t Pin)
 {
 #if defined(CONFIG_SOC_SERIES_STM32WB0X)
  /* On STM32WB0, the PWRC PU/PD control registers should be used instead