mmc: meson-gx: fix dual data rate mode frequencies

In DDR modes, meson mmc controller requires an input rate twice as fast
as the output rate

Fixes: 51c5d844 ("MMC: meson: initial support for GX platforms")
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>