mmc: core: devfreq: migrate to devfreq based clock scaling

This change adds the use of devfreq to MMC.
Both eMMC and SD card will use it.
For some workloads, such as video playback, it isn't
necessary for these cards to run at high speed.
Running at lower frequency, for example 52MHz, in such
cases can still meet the deadlines for data transfers.
Scaling down the clock frequency dynamically has power
savings not only because the bus is running at lower frequency
but also has an advantage of scaling down the system core
voltage, if supported.
Provide an ondemand clock scaling support similar to the
cpufreq ondemand governor having two thresholds,
up_threshold and down_threshold to decide whether to
increase the frequency or scale it down respectively.
The sampling interval is in the order of milliseconds.
If sampling interval is too low, frequent switching of
frequencies can lead to high power consumption and if
sampling interval is too high, the clock scaling logic
would take long time to realize that the underlying
hardware (controller and card) is busy and scale up
the clocks.

Change-Id: I58ddbd93648ded82b304411956e035fb353cd97e
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>
[subhashj@codeaurora.org: fixed trivial merge conflicts & compilation
errors]
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>

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