of: merge of_find_all_nodes() implementations

Merge common code between Microblaze and PowerPC, and make it available
to Sparc

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Michal Simek <monstr@monstr.eu>
Acked-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>