drivers: gpio: nrf: remove GPIO register structure definition
drivers: gpio: nrf: remove GPIO register structure definition

This commit removes the redundant definition of nRF GPIO
register structure that is present in gpio_nrf5.c. The gpio
driver is adapted to use the register definitions directly
from Nordic MDK header files.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

nashif/zephyr,kraj/zephyr,finikorg/zephyr,kraj/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr