mmc: meson-gx: change default tx phase

Initial default tx phase was set to 0 while the datasheet recommends 270.
Some cards fails to initialize with this setting and eMMC mode DDR52 does
not work.

Changing this setting to 270 fixes these issues, without any regression
so far

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