[POWERPC] 4xx: Reorganize 4xx defconfigs

Board specific defconfigs are useful, however with the ability to do
multi-board defconfigs they aren't needed in the top level configs directory.

Move the 4xx board specific defconfigs to individual directories under
arch/powerpc/configs.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>