of: implement of_node_to_nid as a weak function

Implement of_node_to_nid as weak function to remove the dependency on
asm/prom.h. This is in preparation to make prom.h optional.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>