dt-bindings: add compatible string for Allwinner V3s SoC

The compatible string for Allwinner V3s SoC used to be missing.

Add it to the binding document.

Fixes: b074fede ("arm: sunxi: add support for V3s SoC")
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>