pinctrl: sunxi: Add missing H3 R_PIO binding documentation

allwinner,sun8i-h3-r-pinctrl was added by
commit ba83a11104 ("pinctrl: sunxi: Add H3 R_PIO controller support")
but the patch was missing proper binding documentation. This patch fixes
this issue.

Signed-off-by: Krzysztof Adamski <k@japko.eu>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -21,6 +21,7 @@ Required properties:
   "allwinner,sun9i-a80-r-pinctrl"
   "allwinner,sun8i-a83t-pinctrl"
   "allwinner,sun8i-h3-pinctrl"
+  "allwinner,sun8i-h3-r-pinctrl"
 
 - reg: Should contain the register physical address and length for the
   pin controller.