PPC 85xx QEMU: Always assume 1 core

We only need u-boot to bother about a single core in the QEMU machine.
Everything that would require additional knowledge of more cores gets
handled by QEMU and passed straight into the payload we execute.

Because of this setup, it would be counterproductive to enable SMP support
in u-boot. We would have to rip CPUs out of already existing spin tables
and respin them from u-boot. It would be a pretty big mess.

So only assume we have a single core. This fixes errors about CONFIG_MP
being disabled.

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