powerpc/pseries: Fix build break when MEMORY_HOTREMOVE=n

We broke the build when CONFIG_MEMORY_HOTREMOVE=n:

  arch/powerpc/platforms/pseries/hotplug-memory.c:821:8: error: implicit
  declaration of function 'dlpar_memory_readd_by_index'

Add a dummy to fix it.

Fixes: e70d5970

 ("powerpc/pseries: Introduce memory hotplug READD operation")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>