pinctrl: sun6i: Add A31s pinctrl support

The A31s is a stripped down version of the A31, as such it is missing some
pins and some functions on some pins.

The new pinctrl-sun6i-a31s.c this commit adds is a copy of pinctrl-sun6i-a31s.c
with the missing pins and functions removed.

Note there is no a31s specific version of pinctrl-sun6i-a31-r.c, as the
prcm pins are identical between the A31 and the A31s.

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>