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 a4da0d5
"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>

arch/powerpc/include/asm/smp.h

@@ -33,6 +33,7 @@ extern int boot_cpuid;
 extern int spinning_secondaries;
 
 extern void cpu_die(void);
+extern int cpu_to_chip_id(int cpu);
 
 #ifdef CONFIG_SMP
 
@@ -112,7 +113,6 @@ static inline struct cpumask *cpu_core_mask(int cpu)
 }
 
 extern int cpu_to_core_id(int cpu);
-extern int cpu_to_chip_id(int cpu);
 
 /* Since OpenPIC has only 4 IPIs, we use slightly different message numbers.
  *