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

Documentation/devicetree/bindings/misc/allwinner,syscon.txt

@@ -9,6 +9,7 @@ Required properties for the system controller:
 - reg: address and length of the register for the device.
 - compatible: should be "syscon" and one of the following string:
 		"allwinner,sun8i-h3-system-controller"
+		"allwinner,sun8i-v3s-system-controller"
 		"allwinner,sun50i-a64-system-controller"
 		"allwinner,sun8i-a83t-system-controller"