powerpc/mm: Fix build break with BOOK3S_64=n and MEMORY_HOTPLUG=y

The recently merged HPT (Hash Page Table) resize support broke the build
when BOOK3S_64=n (ie. 32-bit or 64-bit Book3E) and MEMORY_HOTPLUG=y:

  arch/powerpc/mm/mem.o: In function `.arch_add_memory':
  (.text+0x4e4): undefined reference to `.resize_hpt_for_hotplug'

Fix it by adding a dummy version.

Fixes: 438cc81a ("powerpc/pseries: Automatically resize HPT for memory hot add/remove")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>