powerpc: Add a 6xx defconfig

This is a defconfig from Dave Jones and should be similar (if not
identical) to the fedora ppc32 defconfig.  The intent is to cover all
cache coherent 6xx based chips and platforms as reasonable.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>