dt-bindings: usb: dwc3: Add disabling LPM for gadget

Add a new DT option to disable LPM for gadget and update the description
for usb2-lpm-disable related to host for clarity.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/f31348ba744318c83b3a9ab1eab75c61122b15ae.1618366071.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>