ARM: sunxi: Introduce Allwinner A20 support

The Allwinner A20 is a dual-core Cortex-A7-based SoC. It is
pin-compatible with the A10, and re-uses most of the IPs found in it,
plus some additional ones like a Gigabit Ethernet controller.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>