tests: drivers: spi: spi_loopback: nrf54h20 cpuapp: add cs gpio

Add CS GPIO to nrf54h20dk/nrf54h20/cpuapp which is connected to
PORT which is in the "fast domain". The fast domain needs to be
resumed from thread context given it requires IPC communication.

This thus ensures that GPIO can be used with and without
PM_DEVICE_RUNTIME enabled.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>