Add support for the GMAC Ethernet controller on Allwinner A20 (sun7i)
processors. Enable for the Cubietruck.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jens Kuske <jenskuske@gmail.com>
Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>