usb: gadget: allow multiple gadgets to be built

now that we have the udc class, we can allow
multiple gadget controller drivers to be
compiled as modules. This will allow for
distro-like kernels for embedded devices.

With this patch, I managed to build an x86
kernel with support for many of the controllers
enabled:

CONFIG_USB_FUSB300=m
CONFIG_USB_R8A66597=m
CONFIG_USB_M66592=m
CONFIG_USB_AMD5536UDC=m
CONFIG_USB_CI13XXX_PCI=m
CONFIG_USB_NET2272=m
CONFIG_USB_NET2280=m
CONFIG_USB_GOKU=m
CONFIG_USB_LANGWELL=m
CONFIG_USB_EG20T=m

Also an ARM kernel with support for many controllers:

CONFIG_USB_FUSB300=m
CONFIG_USB_OMAP=m
CONFIG_USB_R8A66597=m
CONFIG_USB_GADGET_MUSB_HDRC=m
CONFIG_USB_M66592=m
CONFIG_USB_NET2272=m
CONFIG_USB_DUMMY_HCD=m

The next step would be to get rid of the
direct access to arch/ and mach/ directories
on some gadget controllers so that we can
build all of them without depending on their
respective ARCH_* symbols.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>