powerpc/fsl-msi: use a different lockclass for the cascade interrupt

lockdep thinks that it might deadlock because it grabs a lock of the
same class while calling the generic_irq_handler(). This annotation will
inform lockdep that it will not.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>