scsi: ufs: Add sysfs node to dynamically control clock scaling

Provide an option to enable/disable clock scaling during runtime.
Write 1/0 to "clkscale_enable" sysfs node to enable/disable clock
scaling.

Change-Id: I67a6cd317d3dca97a992e240c970719f85067cd8
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>