usb: gadget: udc-core: allow udc class register gadget device

Currently all UDC drivers are calling
device_register() before calling
usb_add_gadget_udc(). In order to avoid
code duplication, we can allow udc-core.c
register that device.

However that would become a really large patch,
so to cope with the meanwhile and allow us
to write bite-sized patches, we're adding
a flag which will be set by UDC driver once
it removes the code for registering the
gadget device.

Once all are converted, the new flag will
be removed.

Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>