usb: gadget: add new usb gadget for ACM and mass storage

This driver provides two functions in one configuration:
a mass storage, and a ACM (serial port) link.
Heavily based on multi.c and cdc2.c

Signed-off-by: Klaus Schwarzkopf <schwarzkopf@sensortherm.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>