powerpc/fsl_msi: change the irq handler from chained to normal

As we do for other fsl-mpic related cascaded irqchips
(e.g. error ints, mpic timers), use a normal irq handler
for msi irqs too.
This brings some advantages such as mask/unmask/ack/eoi
and irq state taken care behind the scenes, kstats
updates a.s.o plus access to features provided by mpic,
such as affinity.

Signed-off-by: Laurentiu Tudor <Laurentiu.Tudor@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>