powerpc: Drop GPL from of_node_to_nid() export to match other arches

The generic implementation of of_node_to_nid() is EXPORT_SYMBOL, added
in commit 298535c0 ("of, numa: Add NUMA of binding
implementation.").

The powerpc implementation added in commit 953039c8 ("[PATCH]
powerpc: Allow devices to register with numa topology") is
EXPORT_SYMBOL_GPL.

This creates an inconsistency for of_node_to_nid() callers across
architectures.

Update the powerpc implementation to be exported consistently with the
generic implementation.

Signed-off-by: Shailendra Singh <shailendras@nvidia.com>
Reviewed-by: Andy Ritger <aritger@nvidia.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>