irqchip/loongson-pch-msi: Prepare get_pch_msi_handle() for AVECINTC

On Loongson-3C6000 and higher systems with AVECINTC irqchip, there may
be multiple PCI segments, but only one PCH-MSI irq domain. In this case,
let get_pch_msi_handle() return the first domain handle.

Co-developed-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Co-developed-by: Liupu Wang <wangliupu@loongson.cn>
Signed-off-by: Liupu Wang <wangliupu@loongson.cn>
Co-developed-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Tianyang Zhang <zhangtianyang@loongson.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240823104337.25577-1-zhangtianyang@loongson.cn