TARGET_NRF5(NRF52_DK):Can't use A5(P0_31) as DigitalOut mode

[zoujixing]

Chech this file : https://github.com/ARMmbed/mbed-os/blob/master/hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/gpio_api.c

The definition "#define GPIO_PIN_COUNT 31"

In function "gpio_init(gpio_t *obj, PinName pin)" will check the pin:
MBED_ASSERT((uint32_t)pin < GPIO_PIN_COUNT); (void) nrf_drv_gpiote_init();

The pin A5 is P0_31(31), if I use A5 as DigitalOut mode, it will printf "mbed assertation failed: (uint32_t)pin < GPIO_PIN_COUNT, file: /extras/mbed-os.lib/hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/gpio_api.c, line 68 "