Add a dtsi file for A31s based boards.

Since the  A31s is the same die as the A31 in a different package, this dtsi
simply includes sun6i-a31.dtsi and then overrides the pinctrl compatible to
reflect the different package, everything else is identical.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>