soc/intel/broadwell/pch: Rename USB files

Done to ease diffing against Lynxpoint.

Change-Id: Ib4280b26799eab6d4a2bb41a14a76695caa31e86
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47021
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/soc/intel/broadwell/pch/Makefile.inc

@@ -33,8 +33,8 @@ romstage-$(CONFIG_DRIVERS_UART_8250MEM) += uart.c
 bootblock-y += usb_debug.c
 romstage-y += usb_debug.c
 ramstage-y += usb_debug.c
-ramstage-y += ehci.c
-ramstage-y += xhci.c
-smm-y      += xhci.c
+ramstage-y += usb_ehci.c
+ramstage-y += usb_xhci.c
+smm-y      += usb_xhci.c
 
 ramstage-srcs += src/mainboard/$(MAINBOARDDIR)/hda_verb.c