tests: spi_controller_peripheral: fix CTRLSEL conflict

Both EXMIF and SPIS peripherals are claiming GPIO port 6 pin 0, but
with different CTRLSEL values. This will trigger a build error with
nrf-regtool 9.0.0 as it is not possible to apply this configuration
to hardware.

Fix the issue by disabling the EXMIF node that is enabled by default.

Signed-off-by: Håkon Amundsen <haakon.amundsen@nordicsemi.no>