dt-bindings: syscon: Add DT bindings documentation for Allwinner V3s syscon

Allwinner V3s SoC has a syscon like the one in H3.

Add its compatible string.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: David S. Miller <davem@davemloft.net>