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>