of/device: Don't register disabled devices

Device nodes with the property status="disabled" are not usable and so
don't register them when parsing the device tree for devices.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Hollis Blanchard <hollis_blanchard@mentor.com>
Cc: Deepak Saxena <deepak_saxena@mentor.com>
Cc: Scott Wood <scottwood@freescale.com>,
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>