mips: dts: realtek: Fold rtl83xx into rtl838x

rtl83xx.dtsi was once (presumably) created as a base for both RTL838x
and RTL839x SoCs. Both SoCs have a different CPU and the peripherals
require different compatibles. Fold rtl83xx.dtsi into rtl838x.dtsi,
currently only supporting RTL838x SoCs, and create the RTL839x base
include later when required.

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