usb: gadget: add some infracture to register/unregister functions

This patch provides an infrastructure to register & unregister a USB
function. This allows to turn a function into a module and avoid the
'#include "f_.*.c"' magic and we get a clear API / cut between the bare
gadget and its functions.
The concept is simple:
Each function defines the DECLARE_USB_FUNCTION_INIT macro whith an unique
name of the function and two allocation functions.
- one to create an "instance". The instance holds the current configuration
  set. In case there are two usb_configudations with one function there will
  be one instance and two usb_functions
- one to create an "function" from the instance.

The name of the instance is used to automaticaly load the module if it the
instance is not yet available.
The usb_function callbacks are slightly modified and extended:
- usb_get_function()
  creates a struct usb_function inclunding all pointers (bind,
  unbind,…). It uses the "instance" to map its configuration. So we can
  have _two_ struct usb_function, one for each usb_configuration.
- ->unbind()
  Since the struct usb_function was not allocated in ->bind() it should
  not kfree()d here. This function should only reverse what happens in
  ->bind() that is request cleanup and the cleanup of allocated
  descriptors.
- ->free_func()
  a simple kfree() of the struct usb_function

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>