include: drivers: gpio: gpio_utils doxygen improvements

add a missing doc + actually show up the utilities from the gpio_utils.h
header file in the doxygen documentation.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

include/zephyr/drivers/gpio/gpio_utils.h

@@ -4,10 +4,20 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/**
+ * @file
+ * @brief Header file for GPIO utility functions
+ * @ingroup gpio_interface
+ */
 
 #ifndef ZEPHYR_INCLUDE_DRIVERS_GPIO_GPIO_UTILS_H_
 #define ZEPHYR_INCLUDE_DRIVERS_GPIO_GPIO_UTILS_H_
 
+/**
+ * @addtogroup gpio_interface
+ * @{
+ */
+
 #include <stdbool.h>
 #include <stdint.h>
 #include <errno.h>
@@ -22,6 +32,12 @@
 extern "C" {
 #endif
 
+/**
+ * @brief Makes a bitmask of allowed GPIOs from a number of GPIOs.
+ *
+ * @param ngpios number of GPIOs
+ * @return the bitmask of allowed gpios
+ */
 #define GPIO_PORT_PIN_MASK_FROM_NGPIOS(ngpios)			\
 	((gpio_port_pins_t)(((uint64_t)1 << (ngpios)) - 1U))
 
@@ -109,4 +125,8 @@ static inline void gpio_fire_callbacks(sys_slist_t *list,
 }
 #endif
 
+/**
+ * @}
+ */
+
 #endif /* ZEPHYR_INCLUDE_DRIVERS_GPIO_GPIO_UTILS_H_ */