mmc: meson-gx: align default phase on soc vendor tree

Align the default Core and Tx phase with the SoC vendor tree.
Even if the Tx phase is different from what the documentation
recommends, it seems to provide better results.

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