soc: arm: nordic: fix GPIO pin forwarder initialization

nRF5340 application core should not attempt to pass control over any
pins to the nRF5340 network core if the latter is disabled.

This commit fixes https://github.com/zephyrproject-rtos/zephyr/issues/43476

.

Signed-off-by: Jedrzej Ciupis <jedrzej.ciupis@nordicsemi.no>