[POWERPC] 83xx/85xx: Reorganize 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 83xx/85xx board specific defconfigs to individual directories under
arch/powerpc/configs.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>