pinctrl: sunxi: Fix irq_of_xlate for the r_pio pinctrl block

The r_pio gpio / pin controller has a pin_base of non 0, we need to
adjust for this before calling sunxi_pinctrl_desc_find_function_by_pin.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>