ANDROID: usb: gadget: configfs: 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.

Bug: 63740241
Bug: 68755607
Bug: 78114713
Bug: 120441124
Change-Id: I0df9ad86ac32d8cdacdea164e9fed49891b45fc2
[badhri: This is a supporting patch for other patches which have
         replacements pipelined. It can be dropped when those
         implementations land.]
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>