of: Add helpers for finding device nodes which have a given property

This commit adds a routine for finding a device node which has a
certain property.  The contents of the property are not taken into
account, merely the presence or absence of the property.

Based on that routine, we add a for_each_ macro for iterating over all
nodes that have a certain property.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>