mmc: meson-gx: use bitfield macros

Use GENMASK consistently for all bit masks and switch to using the
bitfield macros FIELD_GET and FIELD_PREP. This hides parts of the
complexity of dealing with bit fields.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>