On e500 we're basically guaranteed to have 36bits of physical address space
available for our enjoyment. Older chips (like the mpc8544) only had 32bits,
but everything from e500v2 onwards bumped it up.

It's reasonably safe to assume that if you're using the PV machine, your guest
kernel is configured to support 36bit physical address space. So in order to
support more guest RAM, we can move CCSR and other MMIO windows right below the
end of our 36bit address space, just like later SoC versions of e500 do.

With this patch, I'm able to successfully spawn an e500 VM with -m 48G.

Signed-off-by: Alexander Graf <agraf@suse.de>