dt-bindings: irq: imx-irqsteer: Add multi output interrupts support

One irqsteer channel can support up to 8 output interrupts.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: devicetree@vger.kernel.org
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>