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

Provide an option to enable/disable clock gating during runtime.
Write 1 or 0 to "clkgate_enable" sysfs node to enable/disable
clock gating.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>