usb: gadget: Allow function drivers to exist in multiple configurations

USB function drivers are now able to bind to multiple configurations in
the android usb gadget.

In multiple configuration scenario, the usb device exposes to the host
several possible configurations from which the host choose one. Each
configuration is composed of a combination of one or more function
drivers which are the exposed functionality to the host.

In some cases we would like to have a functionality, such as mass storage
device, available in more than one configuration. For example,
we would like the host to have the mass storage functionality in any
configuration it chooses.

This change adds the ability to embed a certain function driver in more
than one configuration, but still, it does not apply that any combination
of configurations will work based on this change. It is known that further
changes are required in some of the function drivers in order to include
them in multiple configurations.

Change-Id: I999c6949888d8e637f216a22fda26b30c233ef7a
CRs-Fixed: 405358
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>