[POWERPC] Make sure smp_processor_id works very early in boot

There's a small period early in boot where we don't know which cpu we're
running on. That's ok, except that it means we have no paca, or more
correctly that our paca pointer points somewhere random.

So that we can safely call things like smp_processor_id(), we need a paca,
so just assume we're on cpu 0. No code should _write_ to the paca before
we've set the correct one up.

We setup the proper paca after we've scanned the flat device tree in
early_setup(), so there's no need to do it again in start_here_common.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>