mmc: core: disable clock scaling before system suspend

This change disables clock scaling in PM notification
for "prepare for system suspend".
This is needed because devfreq creates a dependency
between it and mmc which causes an issue for system suspend.
In this change we break this dependency in earlier stage.

Change-Id: I86dad94c77607b4e8f8fa67035323716f5eb197d
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>