Add binding documentation of TI wrapper for Cadence UFS Controller.

Link: https://lore.kernel.org/r/20191108164857.11466-2-vigneshr@ti.comReviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>