powerpc: Make cpu_to_chip_id() available when SMP=n

Up until now we have only used cpu_to_chip_id() in the topology code,
which is only used on SMP builds. However my recent commit a4da0d50


"Implement arch_get_random_long/int() for powernv" added a usage when
SMP=n, breaking the build.

Move cpu_to_chip_id() into prom.c so it is available for SMP=n builds.

We would move the extern to prom.h, but that breaks the include in
topology.h. Instead we leave it in smp.h, but move it out of the
CONFIG_SMP #ifdef. We also need to include asm/smp.h in rng.c, because
the linux version skips asm/smp.h on UP. What a mess.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>