powerpc/32: Add ppc_defconfig

Add a generic 32-bit defconfig called ppc_defconfig. This means we'll
have a defconfig matching "uname -m" for all cases.

This config is mostly intended for build testing but if someone wants
to tweak it to get it booting on something that would be fine too.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>