irq-bcm2836: Prevent spurious interrupts, and trap them early

irq-bcm2836: Prevent spurious interrupts, and trap them early

The old arch-specific IRQ macros included a dsb to ensure the
write to clear the mailbox interrupt completed before returning
from the interrupt. The BCM2836 irqchip driver needs the same
precaution to avoid spurious interrupts.

Spurious interrupts are still possible for other reasons,
though, so trap them early.