drivers uart_nrfx: Get peripheral address from HAL

Instead of getting the hardcoded address from the DT structure
use its symbolic name which will be resolved by the nRF HAL
definitions to the same value.

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

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