dt-bindings/irqchip/mbigen: add example of MBIGEN generate SPIs

hulk inclusion
category: feature
bugzilla: 46791
CVE: N/A

-------------------------------------------------

Now MBIGEN can support to generate SPIs by writing
GICD_SETSPIR. Add dt example to help document.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Chen Jun <chenjun102@huawei.com>