usb: gadget: udc: amd5536: split core and PCI layer

This patch splits the amd5536udc driver into two -- one that does
pci device registration and the other file that does the rest of
the driver tasks like the gadget/ep ops etc for Synopsys UDC.

This way of splitting helps in exporting core driver symbols which
can be used by any other platform/pci driver that is written for
the same Synopsys USB device controller.

The current patch also includes a change in the Kconfig and Makefile.
A new config option USB_SNP_CORE will be selected automatically when
any one of the platform or pci driver for the same UDC is selected.

Main changes:
- amd5536udc_pci.c: PCI device registration is moved to this file.

- amd5536udc.c:
  This file does rest of the core UDC fucntionality.
  9 symbols are exported so as to be used by amd5536udc_pci.c.
  Module parameter definitions are moved to header file.

- amd5536udc.h:
  Function declarations, module parameters definitions and few common
  header file includes are added to this file

- Kconfig:
  New USB_SNP_CORE option is added which will be auto selected when
  any pci or platform driver config option for the UDC is chosen.

- Makefile:
  Compiles the core and pci files separately.

Signed-off-by: Raviteja Garimella <raviteja.garimella@broadcom.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>