soc: nordic: nrf53: network CPU Force-OFF management

This commit introduces a common API for managing nRF53 SoC network CPU
Force-OFF state.

From the application CPU point of view, the network CPU is a shared
resource used by multiple users. The current solution where every user
controls the network core state directly leads to dependencies between
users and does not scale well.

To address this problem there should be a single entity responsible for
controlling the network CPU Force-OFF signal. This commit introduces
such module.

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