arm64: dts: imx8mm-beacon-baseboard: Correct LED default state

There is no LED default state "none".  leds-gpio driver maps it to
"off", so correct them to fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mm-beacon-kit.dt.yaml:
    leds: led0:default-state:0: 'none' is not one of ['on', 'off', 'keep']
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm-beacon-baseboard.dtsi

@@ -10,19 +10,19 @@ leds {
 		led0 {
 			label = "gen_led0";
 			gpios = <&pca6416_1 4 GPIO_ACTIVE_HIGH>;
-			default-state = "none";
+			default-state = "off";
 		};
 
 		led1 {
 			label = "gen_led1";
 			gpios = <&pca6416_1 5 GPIO_ACTIVE_HIGH>;
-			default-state = "none";
+			default-state = "off";
 		};
 
 		led2 {
 			label = "gen_led2";
 			gpios = <&pca6416_1 6 GPIO_ACTIVE_HIGH>;
-			default-state = "none";
+			default-state = "off";
 		};
 
 		led3 {