clk: sunxi: Rework MMC phase clocks

Instead of having three different clocks for the main MMC clock and the two
phase sub-clocks, which involved having three different drivers sharing the
same register, rework it to have the same single driver registering three
different clocks.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Mike Turquette <mturquette@linaro.org>