dt-binding: irq: imx-irqsteer: Use irq number instead of group number

Not all 64 interrupts may be used in one group. e.g. most irqsteer in
imx8qxp and imx8qm subsystems supports only 32 interrupts.

As the IP integration parameters are Channel number and interrupts number,
let's use fsl,irqs-num to represents how many interrupts supported
by this irqsteer channel.

Note this will break the compatibility of old binding. As the original
fsl,irq-groups was born out of a misunderstanding of the HW config
options and we are not aware of any users of the current binding.
And the old binding was just published in recent months, so it's
worth to change now to avoid confusing in the future.

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>