mmc: meson-gx: enable input clocks

Until now, the mmc clock was left in a good enough state by the ROM
code to be used by the controller. However on some SoC, if the ROM
code finds a bootloader on USB or SPI, it might leave the MMC clock
in state the controller cannot work with.

Enable the input clocks provided to the mmc controller. While the
u-boot mmc controller driver is not doing fancy settings like the Linux,
it at least needs to make these clocks are running.

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