mmc: sdhci: Add host driver support to enable clock gating

Enable config MMC_CLKGATE to enable aggressive clock gating framework that
will disable clocks when the host is not in use for 200ms.

Change-Id: I6bef5dc18b561871689b3d730fd3486323b12520
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
[venkatg@codeaurora.org: sdhci_set_clock is now a library function thats
called from platform clock handler, make changes to address that]
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
[xiaonian@codeaurora.org: fix trivial merge conflict]
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>