powerpc/kconfig: Move CONFIG_PPC32 into Kconfig.cputype

Move CONFIG_PPC32 at the same place as CONFIG_PPC64 for consistency.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6f28085c2a1aa987093d50db17586633bbf8e206.1579024426.git.christophe.leroy@c-s.fr