ARM: dts: bcm281xx: use existing defines for irqs

Use the standard interrupt-controller and ARM GIC constants to
improve the readability of bcm281xx DT irq properties.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Christian Daudt <csd@broadcom.com>