powerpc: Set init_bootmem_done on NUMA platforms as well

For some obscure reason, we only set init_bootmem_done after initializing
bootmem when NUMA isn't enabled. We even document this next to the declaration
of that global in system.h which of course I didn't read before I had to
debug why some WIP code wasn't working properly...

This patch changes it so that we always set it after bootmem is initialized
which should have always been the case... go figure !

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>