powerpc/mpic: Make distribute_irqs obey MPIC_SINGLE_DEST_CPU

Previously we were setting an illegal configuration on mpc85xx
MPICs if CONFIG_IRQ_ALL_CPUS is enabled (which for some reason it is
in mpc85xx_smp_defconfig).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>