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>

tests/drivers/spi/spi_controller_peripheral/boards/nrf54h20dk_nrf54h20_cpuapp_fast_spis.overlay

@@ -78,6 +78,10 @@
 	};
 };
 
+&exmif{
+	status = "disabled";
+};
+
 &gpio0 {
 	status = "okay";
 };