arm/arm64: setup improvements

Handle multiple memory regions being described in the DT. Also
add a more informative error message for when a user does -smp N,
N > NR_CPUS.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

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powerpc will adapt arm's setup to be used as it's own, and it
needs these changes. There's no reason to only do them for power,
so we do them in arm first, getting them ready to be adapted.