of/sparc: move is_root_node() to of.h

Rename is_root_node() to of_node_is_root() and make it available for
all archs to use, as it's not PROM-specific.

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>