arm64: dts: allwinner: h6: add system controller device tree node

As we have already binding for the H6 system controller, add its node
to the device tree.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[fixed compatible string]
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>