ARM: dts: sun6i: Add sun6i-reference-design-tablet.dtsi

Just like with a13/a23/a33 most a31 tablets are derived from the same
reference design. Add a .dtsi file with all the common bits to avoid
endless copy and pasting of these.

The sun6i-reference-design-tablet.dtsi this commit adds is a copy
of sun6i-a31s-colorfly-e708-q1.dts with a few tablet specific bits
removed / left in sun6i-a31s-colorfly-e708-q1.dts.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>