arm64: dts: imx8mm: Remove duplicated machine compatible

Machine compatible string normally is located in board DT, remove
the duplicated one from SoC dtsi.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm.dtsi

@@ -12,7 +12,6 @@
 #include "imx8mm-pinfunc.h"
 
 / {
-	compatible = "fsl,imx8mm";
 	interrupt-parent = <&gic>;
 	#address-cells = <2>;
 	#size-cells = <2>;