mmc: meson-gx: implement voltage switch callback

Implement voltage switch callback (shamelessly copied from sunxi mmc
driver). This allow, with the appropriate tuning function, to use
SD ultra high speed modes.

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