of_node_to_nid() is only relevant in a few architectures.  Don't force
everyone to implement it anyway.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

Reviewed-by: NIngo Molnar <mingo@elte.hu>
Acked-by: NJohn Linn <john.linn@xilinx.com>
Acked-by: NStephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Acked-by: NJohn Williams <john.williams@petalogix.com>
Signed-off-by: NMichal Simek <monstr@monstr.eu>