mmc: meson-gx: remove CLK_DIVIDER_ALLOW_ZERO clock flag

Remove CLK_DIVIDER_ALLOW_ZERO. This flag means that a 1 based divider
with a 0 value will behave as a bypass clock

The mmc divider does not behave like this, a 0 value disables the clock
Remove this flag so CCF never allows a 0 value on this clock

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>