usb: gadget: udc: Rename amd5536udc driver file based on IP

This patch renames the amd5536udc.c that has the core driver
functionality of Synopsys UDC to snps_udc_core.c

The symbols exported here can be used by any UDC driver that uses
the same Synopsys IP.

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