scsi: ufs: change clock gating timeout based on load

Currently we have the same gating timeout for both high load and
low load condition but having the different timeouts would help
to save power in low load condition and increase performance
in high load condition. This change also adds support to tune
these timeouts via sysfs.

Change-Id: I0ac79042d0fd0cd6a6a917ebe3b52db2a1abd0b9
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>