mb/lenovo/x200: Fix docking events

Even though `device` entries are children of `chip` entries in the
devicetree source format, the chips in the translated C structures
are only hooked up to device nodes. Hence, to configure a chip in
a device- or overridetree, it always needs a `device` below it.

This should fix docking events for the X200 ThinkPad.

Change-Id: I561e7ae81f2e096a091868ce51daa1c8f66af067
Signed-off-by: Nico Huber <nico.h@gmx.de>
Found-by: Kevin Keijzer <kevin@quietlife.nl>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51118
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Kevin Keijzer
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/mainboard/lenovo/x200/variants/x200/overridetree.cb

@@ -16,6 +16,7 @@ chip northbridge/intel/gm45
 
 			device pci 1f.0 on # LPC bridge
 				chip ec/lenovo/pmh7
+					device pnp ff.1 on end # dummy
 					register "dock_event_enable" = "0x01"
 				end
 				chip superio/nsc/pc87382