Consolidate of_find_node_by routines

This consolidates the routines of_find_node_by_path, of_find_node_by_name,
of_find_node_by_type and of_find_compatible_device.  Again, the comparison
of strings are done differently by Sparc and PowerPC and also these add
read_locks around the iterations.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Paul Mackerras <paulus@samba.org>
Acked-by: David S. Miller <davem@davemloft.net>