ARM: sunxi: Introduce Allwinner A23 support

The Allwinner A23 is a dual-core Cortex-A7-based SoC. It re-uses most of
the IPs found in previous SoCs, notably the A31.

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