Revert "mmc: core: Remove MMC_CLKGATE"

This reverts commit 9eadcc05.

Clock gating is needed for Qualcomm Platforms hence reverting this
upstream patch.

Change-Id: I96ac0c1c7627e8e5c2d18782e2fc08608f0a7f91
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
[xiaonian@codeaurora.org: fix trivial merge conflict]
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>