usb: gadget: Add function devices to the parent

Added create_function_device to create child
function devices for USB gadget functions.
Android UsbDeviceManager relies on communicating
to the devices created by the gadget functions
to implement functions such as audio_source.

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: I0df9ad86ac32d8cdacdea164e9fed49891b45fc2