mmc: meson-gx: rework tuning function

Rework tuning function of the rx phase. Now that the phase can be
more precisely set using CCF, test more phase setting and find the
largest working window. Then the tuning selected is the one at the
center of the window.

This rework allows to use new modes, such as UHS SDR50

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