ppc64 iSeries: Make stab_initialize() work on iSeries

We don't need to call stab_initialize() for the boot cpu on iSeries, so
we hack around it so that early_setup() can be called on iSeries.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>