ARM: sunxi: dt: Add support for the PineRiver Mini X-plus

This board comes with an Allwinner A10, two external USB ports, a SD
Card reader, 1GB of RAM, the usual video connectors and an onboard wifi
chip.

Of course, the support is quite minimal for now...

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