mb/google/volteer: Add DB_USB3_NO_C option for DB_USB field

Define option value 6 for DB_USB where there is a Type-A port but
no Type-C port on the daughterboard.

BUG=b:151731851
TEST=build volteer boards

Change-Id: I489d24316556dedfecd821e502f1461010b1400f
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46906
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/mainboard/google/volteer/variants/baseboard/devicetree.cb

@@ -6,6 +6,7 @@ fw_config
 		option USB4_GEN3 3
 		option USB3_PASSIVE 4
 		option USB3_NO_A 5
+		option USB3_NO_C 6
 	end
 	field THERMAL 4 7 end
 	field AUDIO 8 10