[SCSI] libsas: Add option for SATA soft reset

This allows a libsas driver to optionally provide a soft reset handler
for libata to drive.  The isci driver allows software to control the
assertion/deassertion of SRST.

[jejb: checkpatch.pl fixes]
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <jbottomley@parallels.com>