mainboard/jetway/nf81-t56n-lf: Turn on PME in devicetree.cb

Change-Id: Ia58994d14ebf488a9200b02ec7af9c71ef4de9e6
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5401
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>