mmc: meson-gx: use CCF to handle the clock phases

Several phases can be controlled on the meson-gx controller, the core, tx
and rx clock phase. The tx and rx uses delays to allow  more fine grained
setting of the phase. To properly compute the phase using delays,
accessing the clock rate is necessary.

Instead of ad-hoc functions, use the common clock framework to set the
clock phases (and access the clock rate while doing it).

Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>