mmc: meson-gx: add device reset

Trigger the reset line of the mmc controller while probing, if available.
The reset should be optional for now, at least until all related DT nodes
have the reset property.

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