irqchip/imx-irqsteer: Change to use reg_num instead of irq_group

One group can manage 64 interrupts by using two registers (e.g. STATUS/SET).
However, the integrated irqsteer may support only 32 interrupts which
needs only one register in a group. But the current driver assume there's
a mininum of two registers in a group which result in a wrong register map
for 32 interrupts per channel irqsteer. Let's use the reg_num caculated by
interrupts per channel instead of irq_group to cover this case.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@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>