Explicitly add gpio write/read prototypes to the HAL header

Author: emilmont

libraries/mbed/hal/gpio_api.h

@@ -29,9 +29,13 @@ extern "C" {
 uint32_t gpio_set(PinName pin);
 
 /* GPIO object */
-void gpio_init(gpio_t *obj, PinName pin, PinDirection direction);
-void gpio_mode(gpio_t *obj, PinMode mode);
-void gpio_dir (gpio_t *obj, PinDirection direction);
+void gpio_init (gpio_t *obj, PinName pin, PinDirection direction);
+
+void gpio_mode (gpio_t *obj, PinMode mode);
+void gpio_dir  (gpio_t *obj, PinDirection direction);
+
+void gpio_write(gpio_t *obj, int value);
+int  gpio_read (gpio_t *obj);
 
 #ifdef __cplusplus
 }