USB: Documentation for USB port power off mechanisms

Describe the mechanisms for controlling port power policy and
discovering the port power state.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
[sarah: wordsmithing]
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
[djbw: updates for device model, hotplug, and rework changes]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>