[POWERPC] Removed config options that we don't support in embedded6xx

When we started arch/powerpc we duplicated a number of config options from
arch/ppc for various platforms that are supported.  Now that we actually
support a few platforms, remove all the ones that haven't been moved over.
Additionally, this cleanup moved the 82xx/PQ2 options over into
arch/powerpc/platforms/82xx/Kconfig where they belong.  It also killed
GEN550 which doesn't exist in arch/powerpc.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>