ARM: sunxi: Add "allwinner,sun6i-a31s" to mach-sunxi

So far the A31s is 100% compatible with the A31, still lets do the same
as what we've done for the A13 / A10s and give it its own compatible string,
in case we need to differentiate later.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[Maxime: Removed unusude CPU_OF_DECLARE_METHOD]
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>