powerpc/powermac: set IRQF_NO_THREAD for xmon/cascade handlers

The xmon and cascade irq handlers must not run as threads.
pmac_pic_lock is already a raw_spinlock, but the irq flag
IRQF_NO_THREAD needs to be set as well.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>