dt-bindings/irq: Add binding for Freescale IRQSTEER multiplexer

This adds the DT binding for the Freescale IRQSTEER interrupt
multiplexer found in the i.MX8 familiy SoCs.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>