[POWERPC] move 82xx/83xx/86xx Kconfig options to platform selection

The cores used in the MPC82xx/83xx/86xx embedded controllers are very similar
to those in the 32 bit general-purpose processors, so it makes sense to
treat them as the same CPU family.

Choosing between the embedded platforms and the multiplatform code is
now done in the platform menu, but functionally everything stays the
same.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>