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

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8NULW


CVE: N/A

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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>
Signed-off-by: Yuntao Liu <liuyuntao12@huawei.com>