[SCSI] isci: enable clock gating

Enabling clock gating for power savings on entry to controller ready
state. Disable SCU clock gating for power savings on exit from the
controller ready state.

The gating is fully automated by silicon after setting the mode.

Signed-off-by: Marcin Tomczak <marcin.tomczak@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>