Add M4 device tree overlay

- Add an overlay for using the M4. This simply removes control of uart3
from the AP, so that the MCU can utilize it.

Change-Id: Iae56aad112465a22bebe2192f1f0bd096b098b42
diff --git a/arch/arm64/boot/dts/freescale/Makefile b/arch/arm64/boot/dts/freescale/Makefile
index 97db543..b0c09a9 100644
--- a/arch/arm64/boot/dts/freescale/Makefile
+++ b/arch/arm64/boot/dts/freescale/Makefile
@@ -117,6 +117,7 @@
 				 fsl-imx8mq-evk-dp.dtb \
 				 fsl-imx8mq-evk-edp.dtb \
 				 fsl-imx8mq-phanbell.dtb \
+				 fsl-imx8mq-phanbell-m4.dtb \
 				 fsl-imx8mq-yorktown.dtb
 dtb-$(CONFIG_ARCH_FSL_IMX8MM) += fsl-imx8mm-columbia.dtb \
 				 fsl-imx8mm-evk.dtb \
diff --git a/arch/arm64/boot/dts/freescale/fsl-imx8mq-phanbell-m4.dts b/arch/arm64/boot/dts/freescale/fsl-imx8mq-phanbell-m4.dts
new file mode 100644
index 0000000..604cd79d
--- /dev/null
+++ b/arch/arm64/boot/dts/freescale/fsl-imx8mq-phanbell-m4.dts
@@ -0,0 +1,12 @@
+/dts-v1/;
+/plugin/;
+
+/ {
+	compatible = "fsl,imx8mq-phanbell", "fsl,imx8mq";
+	fragment@0 {
+		target = <&uart3>;
+		__overlay__ {
+			status = "disabled";
+		};
+	};
+};