ARM: sunxi: Add Allwinner A23 dtsi

The Allwinner A23 is a tablet oriented SoC with 2 Cortex-A7 cores
and a Mali-400MP2 GPU.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>