This definition was needed to translate the old symbol used in MDK
for accessing the GPIO peripheral in nRF51 SoCs (NRF_GPIO) to the
one used for all newer SoCs (NRF_P0). Now the translation is done
in the HAL for GPIO (nrf_gpio.h).
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
