dt-bindings: add compatible string for Allwinner R40 pinctrl

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>

Documentation/devicetree/bindings/pinctrl/allwinner,sunxi-pinctrl.txt

@@ -22,6 +22,7 @@ Required properties:
   "allwinner,sun8i-a83t-pinctrl"
   "allwinner,sun8i-h3-pinctrl"
   "allwinner,sun8i-h3-r-pinctrl"
+  "allwinner,sun8i-r40-pinctrl"
   "allwinner,sun50i-a64-pinctrl"
   "allwinner,sun50i-a64-r-pinctrl"
   "allwinner,sun50i-h5-pinctrl"