pinctrl: sunxi: add reset control support

The A31 SoC define a reset line for the R_PIO block which needs to be
deasserted.

Try to retrieve a reset control and deassert if one was found.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>