usbdebug: Refactor descriptor probing of dongle

Organized such that it is easy to support devices that do not
export special Debug Descriptor. Some of these can still work
in a fixed configuration and/or require additional initialisation
for UART clocks etc.

Change-Id: Id07fd6b69007332d67d9e9a456f58fdbca1999cd
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7209
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins)

src/drivers/usb/ehci_debug.h

@@ -37,6 +37,11 @@ void ehci_debug_select_port(unsigned int port);
 #define DBGP_EP_BUSY		(1<<2)
 #define DBGP_EP_STATMASK	(DBGP_EP_VALID | DBGP_EP_ENABLED)
 
+#define DBGP_MAX_ENDPOINTS	4
+#define DBGP_SETUP_EP0		0	/* Compulsory endpoint 0. */
+#define DBGP_CONSOLE_EPOUT	1
+#define DBGP_CONSOLE_EPIN	2
+
 struct ehci_dbg_port;
 
 struct dbgp_pipe