sparc: break out some PROM device-tree building code out into drivers/of

Transitioning into making this useful for architectures other than sparc.
This is a verbatim copy of all functions/variables that've been moved.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>