ARM: dts: sun7i: Add Merrii A20 Hummingbird board

This adds support for the A20 Hummingbird:
http://www.merrii.com/en/pla_d.asp?id=171

This patch enable most on-board peripherals supported on current kernel,
such as uart, i2c, spi, pwm, ohci/ehci, gmac and mmc.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Wills Wang <wills.wang.open@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>