ARM: sunxi: GMAC driver phy interface based on config

GMAC in A20 SoC supports MII and RGMII modes.
Support both so we can use GMAC in different boards.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
ARM: sunxi: Do not select unused GMAC pins under RGMII mode

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
ARM: sunxi: Let cubieboard2 use the GMAC driver

Signed-off-by: Chen-Yu Tsai <wens@csie.org>