mips: dts: realtek: Add RTL9302C board

Add support for the RTL9302 SoC and the RTL9302C_2xRTL8224_2XGE
reference board.

The RTL930x family of SoCs are Realtek switches with an embedded MIPS
core (800MHz 34Kc). Most of the peripherals are similar to the RTL838x
SoC and can make use of many existing drivers.

Add in full DSA switch support is still a work in progress.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>