USB: defines for USB "Link Power Management" (LPM) ECN

There's a new PM-related change notice for the USB 2.0 specification
called "Link Power Management" (LPM).  It defines a new "L1 Suspend"
state which resembles the current (L2) suspend state, except that it
can be entered and exited much more quickly.  It should thus be more
useful for runtime PM, even though it doesn't mandate reduced power
draw from VBUS.

This patch provides the relevant #defines for usbcore.  Actually
implementing these mechanisms requires host silicon that can generate
new USB packets, plus hubs handling some new requests and peripherals
which understand the new packets.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>