lenovo/t520: fix devicetree

SATA Port documentation
PCIe unused ports and documentation
T520 have no keyboard backlight

Change-Id: I517ff8519ea22a9a7a9b6e3136efd15d4a0f8fc4
Signed-off-by: Nicolas Reinecke <nr@das-labor.org>
Reviewed-on: http://review.coreboot.org/6743
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>