powerpc/pseries: Fix build break for SPLPAR=n and CPU hotplug

Commit e67e02a5 ("powerpc/pseries: Fix cpu hotplug crash with
memoryless nodes") adds an unconditional call to
find_and_online_cpu_nid(), which is only declared if CONFIG_PPC_SPLPAR
is enabled. This results in the following build error if this is not
the case.

  arch/powerpc/platforms/pseries/hotplug-cpu.o: In function `dlpar_online_cpu':
  arch/powerpc/platforms/pseries/hotplug-cpu.c:369:
  			undefined reference to `.find_and_online_cpu_nid'

Follow the guideline provided by similar functions and provide a dummy
function if CONFIG_PPC_SPLPAR is not enabled. This also moves the
external function declaration into an include file where it should be.

Fixes: e67e02a5

 ("powerpc/pseries: Fix cpu hotplug crash with memoryless nodes")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
[mpe: Change subject to emphasise the build fix]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>