mips: dts: realtek: add device_type property to cpu node

Add device_type = "cpu" to the cpu node for the rtl838x SoC. This
resolves the following dtbs_check complaint:

 cpus: cpu@0: 'cache-level' is a required property
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/realtek/rtl838x.dtsi

@@ -6,6 +6,7 @@ cpus {
 		#size-cells = <0>;
 
 		cpu@0 {
+			device_type = "cpu";
 			compatible = "mips,mips4KEc";
 			reg = <0>;
 			clocks = <&baseclk 0>;