powerpc: Factoring mpic cpu id fetching into a function

The following code snippet:

	unsigned int cpu = 0;
	if (mpic->flags & MPIC_PRIMARY)
		cpu = hard_smp_processor_id();

is seen in several places in the 'mpic.c' code.  This changeset factors
that pattern out into a helper function called 'mpic_processor_id'.

Signed-off-by: Meador Inge <meador_inge@mentor.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>