pinctrl: sunxi: Add irq pinmuxing to sun6i "r" pincontroller

Add pinmuxing for external interrupt functionality through the
sun6i "r" pincontroller.

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>