Split out common parts of prom.h

This creates linux/of.h and includes asm/prom.h from it.

We also include linux/of.h from asm/prom.h while we transition.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Paul Mackerras <paulus@samba.org>
Acked-by: David S. Miller <davem@davemloft.net>