mmc: mtk-sd: select REGULATOR

The mtk-sd driver requires a regulator to be present, even if it is
the "fixed" regulator.  So select REGULATOR to make it hard to build
unusable configurations.

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