irqchip/irq-bcm2836: Add support for DT interrupt polarity

In order to properly define the polarity of the per-cpu interrupts,
we need to support for a second property cell. But this must be
optional to keep backward compatibility with old DT blobs.

Suggested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>