dt-bindings/bcm283x: Define polarity of per-cpu interrupts

This patch define the polarity of the per-cpu interrupts on BCM2836
and BCM2837 in order to avoid the warnings from ARM arch timer code:

    arch_timer: WARNING: Invalid trigger for IRQ19, assuming level low
    arch_timer: WARNING: Please fix your firmware
    arch_timer: cp15 timer(s) running at 19.20MHz (virt).

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