dt-bindings: Support turning off UFS's Power Supplies

Add documentation for the vccq-pwr-collapse-sup and
vccq2-pwr-collapse-sup. If the system supports turning
off UFS's power supplies during system power collapse events,
enabling the feature will turn off the UFS's power supplies
during System Suspend events. During System Resume events,
the UFS power supplies will be turned back on again and the
UFS device will be re-initialized.

Change-Id: I38fd1209bab6c7b008b4b10dc4993ed29ad87727