ARM: dts: rockchip: Add dtb for the Radxa Rock 2 Square board

The Radxa Rock 2 Square board is a combination of the Radxa Rock 2 SoM
with the Square baseboard. Add a dtsi for the SoM which can be included
into the dts for the various baseboards (e.g. full and square) and a dts
for the square board.

Currently supported are serial console, wired networking, hdmi output,
eMMC and SD storage and USB.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>