clk: meson: reset mmc clock on probe

On some SoCs, depending on the boot device, the MMC clock block may be
left in a weird state by the ROM code, in which no decent clock may be
provided. Reset the related register to make sure a sane MMC clock is
ready for the controller.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>