mmc: cmdq: add clock scaling for CMDQ mode

This change adds clock scaling ability to command-queueing
mode, it does so by adding next logic:

* Statistics collection for CMDQ data path
* Empty the queue and Halt it before frequency change
* Scale from data path in case host claiming is not possible

Change-Id: I53a323b55df4d7c27e3ee3426ee4e856e533522c
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>