scsi: ufshcd: Let vendor override devfreq parameters

Vendor drivers may have a need to update the polling interval and
thresholds.  Provide a vops for vendor drivers to use.

Link: https://lore.kernel.org/r/acd79e00396cff855256adad47f615ccdbde85ac.1585160616.git.asutoshd@codeaurora.org


Acked-by: Avri Altman <Avri.Altman@wdc.com>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>