powerpc/mm: Assume first cpu is boot_cpuid not 0

arch/powerpc/mm/mmu_context_nohash.c assumes the boot cpu
will always have smp_processor_id() == 0. This patch fixes
that assumption

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>