powerpc/pseries: Fix compile of memory hotplug without CONFIG_MEMORY_HOTREMOVE

51925fb3 "powerpc/pseries: Implement memory hotplug remove in the kernel"
broke compile when CONFIG_MEMORY_HOTREMOVE is not defined due to missing
symbols. This fixes the issue by adding the missing symbols.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>