sunxi: Add A20-SOM204-EVB-eMMC board

A20-SOM204 board has option with onboard 16GB eMMC. The chip is wired
to MMC2 slot.

This patch adds defconfig and dts files for this board. The dts is same
with mainline kernel.

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>