powerpc/config is shared by powerpc, powerpcspe and ppc64, so move
it to kernelarch-powerpc/config where it will be automatically used
for all of them.
powerpc/config.powerpc64 is also used by ppc64, so move it to
kernelarch-powerpc/config-arch-64.
powerpcspe has only one flavour, so rename its config file to config.
svn path=/dists/trunk/linux/; revision=21265