powerpc: Add includes to fix powernv/rng.c build

Caused by commit a4da0d50 ("powerpc: Implement
arch_get_random_long/int() for powernv") from the powerpc tree
interacting with commit b5b4bb3f ("of: only include prom.h on sparc")
from the dt-rh tree.

I added this merge fix patch (which will need to be sent to Linus when
these two trees get merged, or could be applied now to the powerpc tree):

[ Also add linux/smp.h to get cpu_to_chip_id -- BenH ]

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>