env: mmc: depends on the MMC framework

The raw MMC environment directly calls into the MMC framework. Make sure
it's enabled before we can select it.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>