powerpc: Exporting boot_cpuid_phys

Kernel loadable module can use hard_smp_processor_id() if building with SMP
kernel. In order to make it work for UP kernels too, boot_cpuid_phys
symbol (which is what hard_smp_processor_id() macro resolves to
in non-SMP configuration) must be exported.

Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>