ARM: sunxi: Add gmac clock macros

Add macros for gmac clock control, instead of using raw values.
Also add comment noting GMAC ahb gate is in second gate register,
to avoid confusion over same offset as EMAC.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
ARM: sunxi: Move GMAC init code into seperate glue layer driver

GMAC init code now exists in a glue layer driver under drivers/net,
instead of sunxi board initialization code. It has been cleaned up
to use macros for clock controls.

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