USB: gadget: Add a multi function gadget ...

Similar to the Kernel multi function this gadget driver is used
for creating a USB device with multiple functions. This is
created and removed with the newly created 'usbgadget' command.
Based on the options it creates combinations of DFU, fastboot
and serial USB functions.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>