pinctrl: sun6i: Add some missing functions

While working on pinctrl for the A31s, I noticed that function 4 of
PA15 - PA18 was missing, add these.

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