powerpc: Make ppc32 respect the boot cpu id for !CONFIG_SMP

Previously the FDT header field boot_cpuid_phys wasn't actually used
on ppc32.  Instead the physical boot cpuid was assumed to be 0 for
!CONFIG_SMP.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>