clk: sunxi: Move the GMAC clock to a file of its own

Since we have a folder of our own, we can actually make use of it by
splitting the huge clock file into several sub drivers.

The gmac clock is pretty easy to deal with, since it's pretty much
isolated and doesn't have any dependency on the other clocks.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Emilio López <emilio@elopez.com.ar>