arm: sunxi: add support for V3s SoC

Allwinner V3s is a low-end single-core Cortex-A7 SoC, with 64MB
integrated DRAM, and several peripherals.

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