powerpc/e500: add paravirt QEMU platform

This gives the kernel a paravirtualized machine to target, without
requiring both sides to pretend to be targeting a specific board
that likely has little to do with the host in KVM scenarios.  This
avoids the need to add new boards to QEMU just to be able to
run KVM on new CPUs.

As this is the first platform that can run with either e500v2 or
e500mc, CONFIG_PPC_E500MC is now a legitimately user configurable
option, so add a help text.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>