arm64: dts: rockchip: Add support for NanoPi R6C

NanoPi R6C is mostly same as R6S variant. It has M2 port instead of a
NIC port and different led labeling.

Signed-off-by: Muhammed Efe Cetin <efectn@protonmail.com>
Link: https://lore.kernel.org/r/0f9ee0baa6c9de4d54dd6d13957ca15a63ec934f.1703934548.git.efectn@protonmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>