USB: Composite framework: Add suspended sysfs entry

This patch adds a sysfs entry (/sys/devices/platform/_UDC_/gadget/suspended) to
show the suspend state of an USB composite gadget.

Signed-off-by: Fabien Chouteau <fabien.chouteau@barco.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>