[SCSI] Documentation/devicetree: Add DT bindings for UFS host controller

Compatible list is used in commit 03b1781a but is not documented.
Add necessary device tree bindings to describe on-chip UFS host
controllers.

Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Santosh Y <santoshsy@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>