ARM: sunxi: add support for R40 SoC

Allwinner R40 is a new SoC, with Quad Core Cortex-A7 and peripherals
like A20.

Add support for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>