pinctrl: sunxi: add of_xlate function

Since the pin controller of sunxi chips is represented as a
single bank in the driver.

Since this is neither convenient nor represented that way in the
datasheets, define a custom of_xlate function with the layout
<bank pin flag>

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