ARM: dts: sunxi: Add Olimex A20-SOM204-EVB-eMMC board

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

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