sunxi: Fix gmac not working on the Colombus board

The phy is using a RGMII interface, which we need to specify in our
board-config, and the dts needs a gmac section (the dts changes have
also been submitted to the kernel).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>