device property: Allow iterating over available child fwnodes

Implement a new helper function fwnode_get_next_available_child_node(),
which enables obtaining next enabled child fwnode, which
works on a similar basis to OF's of_get_next_available_child().

This commit also introduces a macro, thanks to which it is
possible to iterate over the available fwnodes, using the
new function described above.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>