ARM: sun7i: Add Olimex A20-Olinuxino-Micro support

The Olimex A20-Olinuxino is an open-hardware board based on the
Allwinner A20 SoC, with most of the pins exported on headers, a 10/100M
ethernet port, SATA, SD and uSD slots, etc.

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