pinctrl: sunxi: Add missing H3 R_PIO binding documentation

allwinner,sun8i-h3-r-pinctrl was added by
commit ba83a111 ("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>