mmc: core: add clock-scaling support to HS400 cards

This patch adds clock scaling support to HS400 cards.
Scaling down to 52MHz from HS400 involves:
 - switching the bus-speed mode to HS at 52MHz

Scaling up to HS400 would require all of the initialization
process upto HS400 mode selection.

Change-Id: I8196d6666bcc0ef327659253df53a17792fa51f7
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>