Allwinner R40 has a pin controller like the ones in older Allwinner SoCs
(especially A20), and can use modified version of the A10/A20 pinctrl
driver.

Add a compatible string for it.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>