usbdebug: Move under drivers/usb

Also relocate and split header files, there is some interest
for EHCI debug support without PCI.

Change-Id: Ibe91730eb72dfe0634fb38bdd184043495e2fb08
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5129
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>
diff --git a/src/drivers/Makefile.inc b/src/drivers/Makefile.inc
index 34ae62e..4b63a8f 100644
--- a/src/drivers/Makefile.inc
+++ b/src/drivers/Makefile.inc
@@ -29,6 +29,7 @@
 subdirs-y += realtek
 subdirs-y += sil
 subdirs-y += trident
+subdirs-y += usb
 subdirs-y += ics
 subdirs-y += spi
 subdirs-y += ti