scsi: ufs: Add document for vcc-low-voltage-sup

UFS3.0 devices are capable of running at lower level (2.5v)
of Vcc rail. If both UFS host and device support 2.5v Vcc level,
this vcc-low-voltage-sup is used to specify the support for
this feature.

Change-Id: I598af391a4c6a04a49910b17247611af28231b74
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>