mmc: meson-gx: clean up some constants

Remove unused clock rate defines. These should not be defined but
requested from the clock framework.

Also correct typo on the DELAY register

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