usb: gadget: Allow a non-SuperSpeed gadget to support LPM

This commit allows a gadget that does not support SuperSpeed to indicate
that it supports LPM. It does this by setting the 'lpm_capable' flag in
the gadget structure.

If a gadget sets this, the composite gadget framework will set the
bcdUSB to 0x0201 to indicate that this supports BOS descriptors, and
also return a USB 2.0 Extension descriptor as part of the BOS descriptor
set.

See USB 2.0 LPM ECN Section 3.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Sevak Arakelyan <sevaka@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>