scsi: ufs: Add sysfs node to dynamically control hibern8 on idle

Provide an option to enable/disable hibern8 on idle functionality during
runtime. Write 1 or 0 to "hibern8_on_idle_enable" sysfs node to
enable/disable hibern8 on idle functionality.

Change-Id: Id4b6253c3c53ed71575c05596abbd4dd99821eff
[subhashj@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
[venkatg@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>