mmc: core: add runtime PM voting to devfreq context

This change adds usage for runtime PM for devfreq
context.
In case clock scaling is done from devfreq context its
best to vote for runtime PM to avoid race between
scaling and PM.

Change-Id: I6fc7e2236b44b1f3a322da36552a7f9237c4dac6
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>