powerpc/ps3: Move ps3_mm_add_memory to device_initcall

Change the PS3 hotplug memory routine ps3_mm_add_memory() from
a core_initcall to a device_initcall.

core_initcall routines run before the powerpc topology_init()
startup routine, which is a subsys_initcall, resulting in
failure of ps3_mm_add_memory() when CONFIG_NUMA=y.  When
ps3_mm_add_memory() fails the system will boot with just the
128 MiB of boot memory

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>