of: fix recursive locking in of_get_next_available_child()

of_get_next_available_child() acquires devtree_lock, then calls
of_device_is_available() which calls of_get_property() which calls
of_find_property() which tries to re-acquire devtree_lock, thus causing
deadlock.

To avoid this, create a new __of_device_is_available() which calls
__of_get_property() instead, which calls __of_find_property(), which
does not take the lock,. Update of_get_next_available_child() to call
the new __of_device_is_available() since it already owns the lock.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>