USB: gadget: Split Target based USB-Gadget driver (tcm_usb_gadget.c)

Split Target based USB-Gadget driver (tcm_usb_gadget.c) into composite
driver (tcm_usb_gadget.c) and Function driver (f_tcm.c).  This is
required for other composite drivers like Android to include Target
based USB function (UASP) along with other functions.

Composite drivers can include f_tcm.c like any other function and
register a callback function.  This callback function is called
when Target Fabric is enabled via configfs.  This callback can be
used to enable enumeration with host.

Change-Id: I36beea07bb3641a224694c157849ecec347d5943
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>