mmc: mtk-sd: enable internal card-detect logic.

The mtk-sd silicon has integrated card-detect logic that is
enabled on the MT7621.  The circuit is phased out on newer hardware so
we should be careful to only enabled it on hardware known to support
it.  This a new "use_internal_cd" flag in struct mtk_mmc_compatible.

If the sdhci isn't marked non-removable and doesn't have a
cd-gpio configured, and if use_internal_cd is set, then assume the
internal cd logic should be used as recommended by
 Documentation/devicetree/bindings/mmc/mmc.txt

Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>