usb: Allow registration of a single non-contiguous string descriptor for WinUSB

Classic WinUSB support is detected by probing for a string descriptor
at index 0xEE with a special string.
usbd_register_extra_string() allows registration of a string at this
index without having to provide 237 other string descriptors

Originally filed as https://github.com/libopencm3/libopencm3/pull/849

WCID reference: https://github.com/pbatard/libwdi/wiki/WCID-Devices

lib/usb/usb_private.h

@@ -92,6 +92,10 @@ struct _usbd_device {
 
 	const struct _usbd_driver *driver;
 
+	/* Extra, non-contiguous user string descriptor index and value */
+	int extra_string_idx;
+	const char* extra_string;
+
 	/* private driver data */
 
 	uint16_t fifo_mem_top;