drivers gpio_nrfx: Get peripheral address from HAL

Instead of getting the hardcoded address from the DT structure
use its symbolic name (Also from DT) which will be resolved
by the nRF HAL definitions.

This allows the GPIO peripherals' addresses to be redefined
for the simulated targets.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>