powerpc/fsl_msi: mark the msi cascade handler IRQF_NO_THREAD

The commit 543c043c ("powerpc/fsl_msi: change the irq handler from
chained to normal") changes the msi cascade handler from chained to
normal. Since cascade handler must run in hard interrupt context, this
will cause kernel panic if we force threading of all the interrupt
handler via kernel command parameter 'threadirqs'. So mark the irq
handler IRQF_NO_THREAD explicitly.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>