mmc: core: Allow changing bus frequency for SD/eMMC cards in runtime

Currently, bus frequency is set during the card initialization and
never changed until a new card is inserted. In some low power use
cases, scaling the clock frequencies while the card is in transfer
state would allow power savings. This change allows bus frequency
to be changed after the card initialization.

Change-Id: Iac064221199f69d162d91f5311becd735c15700a
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>