[nrf fromlist] tests: drivers: spi: controller_periph: lower nrf7120 spi22 freq

The nrf7120 SPIM22 instance does not support 16MHz, lower to 8MHz.

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

Upstream PR #: 108327

Author: bjarki-andreasen

tests/drivers/spi/spi_controller_peripheral/boards/nrf7120dk_nrf7120_cpuapp.overlay

@@ -58,7 +58,7 @@
 	dut_spi_dt: test-spi-dev@0 {
 		compatible = "vnd,spi-device";
 		reg = <0>;
-		spi-max-frequency = <DT_FREQ_M(16)>;
+		spi-max-frequency = <DT_FREQ_M(8)>;
 	};
 };