clk: sunxi: Add Allwinner A20/A31 GMAC clock unit

The Allwinner A20/A31 clock module controls the transmit clock source
and interface type of the GMAC ethernet controller. Model this as
a single clock for GMAC drivers to use.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Emilio López <emilio@elopez.com.ar>