tests: drivers: spim_mosi_toggles: frequency for nRF92

SPIM instance used in the test has 16 MHz base clock, so
prescaler for 1.28 MHz could not be found. Use 1 MHz,
just like other slow instances.

Signed-off-by: Michał Stasiak <michal.stasiak@nordicsemi.no>

Author: mstasiaknordic

tests/drivers/spi/spim_mosi_toggles/boards/nrf9251dk_nrf9251_cpuapp.overlay

@@ -59,6 +59,6 @@
 	dut_spi_dt: test-spi-dev@0 {
 		compatible = "vnd,spi-device";
 		reg = <0>;
-		spi-max-frequency = <1280000>;
+		spi-max-frequency = <DT_FREQ_M(1)>;
 	};
 };