[POWERPC] PS3: Fix memory hotplug

A change was made to walk_memory_resource() in commit
4b119e21

 that added a
check of find_lmb().  Add the coresponding lmb_add()
call to ps3_mm_add_memory() so that that check will
succeed.

This fixes the condition where the PS3 boots up with
only the 128 MiB of boot memory, and doesn't see the
other 128MiB that is available.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>