dt-bindings: pinctrl: add missing compatible string for V3s

The pinctrl driver of V3s is already available and used in the kernel,
but the compatible string of it is forgotten to be added.

Add the missing compatible string.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>