of/numa: drop export of of_node_to_nid

The "generic" implementation of of_node_to_nid is only used by
arm64 and only in built-in code, so remove its export. Any
device with a struct device should be able to use dev_to_node()
instead. Also, exporting of_node_to_nid doesn't actually work if
we build a module on an arch that doesn't select OF_NUMA nor provide its
own of_node_to_nid implementation.

Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>