mb/system76/whl-u: Add missing device node

All `chip` entries need a device node below them to actually get hooked
up. Add a dummy generic device like other instances of this driver use.

Change-Id: Ifbb5c9a6b389a2c809ce654d584d5197af764893
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57475
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Crawford <tcrawford@system76.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/system76/whl-u/devicetree.cb

@@ -90,6 +90,7 @@ chip soc/intel/cannonlake
 		device pci 14.3 on      # CNVi wifi
 			chip drivers/wifi/generic
 				register "wake" = "PME_B0_EN_BIT"
+				device generic 0 on end
 			end
 		end
 		device pci 14.5 off end # SDCard