usb: make usb port a real device

This patch turns each USB port on a hub into a new struct device.  This
new device has the USB hub interface device as its parent.  The port
devices are stored in a new structure (usb_port), and an array of
usb_ports are dynamically allocated once we know how many ports the USB
hub has.

Move the port_owner variable out of usb_hub and into this new structure.

A new file will be created in the hub interface sysfs directory, so
add documentation.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>