arm64: dts: rockchip: Add DT for NanoPi M4

There are a number of subtle differences between the nanopi4 variants,
and where they disagree, the common DTSI currently follows the details
of NanoPi M4. In order to improve matters even more, let's add a
separate DTS for the M4 to which we can start splitting things out
appropriately. The third variant, NanoPi NEO4, is a lot closer to the M4
than either is to the larger T4, so arguably could get away with just
sharing the M4 DT for now (plus I have neither of the smaller boards to
actually test with).

CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>