mips: dts: realtek: Decouple RTL930x base DTSI

The RTL930x SoC series is sufficiently different to warrant its own base
dtsi. This ensures no properties need to be deleted or overwritten, and
prevents accidental inclusions of updates from rtl83xx.dtsi.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # For RTL9302C
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>