of: base: Add of_get_next_child_with_prefix() stub

of: base: Add of_get_next_child_with_prefix() stub

1fcc67e3a354 ("of: base: Add for_each_child_of_node_with_prefix()") added
of_get_next_child_with_prefix() but did not add a stub for the !CONFIG_OF
case.

Add a of_get_next_child_with_prefix() stub so users of
for_each_child_of_node_with_prefix() can be built for compile testing even
when !CONFIG_OF.

Fixes: 1fcc67e3a354 ("of: base: Add for_each_child_of_node_with_prefix()")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>