powerpc: Fix irq distribution

setting CONFIG_IRQ_ALL_CPUS distributes IRQs to CPUs only when
the number of online CPUs equals NR_CPUS.  See commit
280ff97494e0fef4124bee5c52e39b23a18dd283 "sparc64: fix and
optimize irq distribution" for more details.

Using the online mask fixes IRQ-to-CPU distribution on systems
that boot with less than NR_CPUS.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>