sunxi: Fix gmac not working reliable on the Bananapi

In order for the gmac nic to work reliable on the Bananapi, we need to poke
these 2 undocumented bits in the gmac clk register. Since these are
undocumented, this commit only sets these bits on the Bananapi for now.

I'll contact Allwinner to try and get these bits documented, once they
are documented we can hopefully replace this hack with a better patch.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>