dt-bindings/bcm2836-l1-intc: Add interrupt polarity support

This increases the interrupt cells for the 1st level interrupt controller
binding in order to describe the polarity like on the other ARM platforms.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>