mmc: disable the mmc clock during power off

There is no point in having the mmc clock enabled during
power off. Disable the mmc clock. This is similar to how it's
programmed in Linux Kernel.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>