usb: gadget: Add helper macro for usb_composite_driver boilerplate

Introduce the module_usb_composite_driver macro as a convenience macro
for USB gadget composite driver modules, similar to module_usb_driver.
It is intended to be used by drivers which init/exit section does
nothing but calling usb_composite_probe/usb_composite_unrregister. By
using this macro it is possible to eliminate a few lines of boilerplate
code per USB gadget composite driver.

Based on f3a6a4b6 ("USB: Add helper macro for usb_driver boilerplate")
which introduced the according macro for USB drivers.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Felipe Balbi <balbi@ti.com>