w83627hf: add method to retrieve wake event source register to ASL include

w83627hf: add method to retrieve wake event source register to ASL include

Add a method WAKS to devtree.asl which returns the wake-up source register
to simplify retrieving the wake source e.g. in \_WAK.

Change-Id: Ia258f8fc9ff79b18391c55464da73863889e2255
Signed-off-by: Christoph Grenz <christophg+cb@grenz-bonn.de>
Reviewed-on: http://review.coreboot.org/297
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>