scsi: ufs: Add support for clock scaling using devfreq framework

The clocks for UFS device will be managed by generic DVFS (Dynamic
Voltage and Frequency Scaling) framework within kernel. This devfreq
framework works with different governors to scale the clocks. By default,
UFS devices uses simple_ondemand governor which scales the clocks up if
the load is more than upthreshold and scales down if the load is less than
downthreshold.

Change-Id: I1c3944a5b1771b967c1df3e441a75fd584d6e2a5
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>