arm64: dts: rockchip: move rk3568 dtsi to rk356x dtsi

In preparation for separating the rk3568 and rk3566 device trees, move
the base rk3568 dtsi to rk356x dtsi.
This will allow us to strip out the rk3568 specific nodes.
Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Link: https://lore.kernel.org/r/20210710151034.32857-2-pgwipeout@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>