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>