ARM: sunxi: Add the Allwinner A31 compatible to the machine definition

The Allwinner A31 is a quad-Cortex-A7 based SoC, which shares a lot of
IPs with the previous SoCs from Allwinner, like the PIO, I2C, UARTs,
timers, watchdog IPs, but also differs by dropping the WEMAC ethernet
controller and most notably dropping the in-house IRQ controller in
favor of a ARM GIC one.

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