irqchip/loongson-pch-lpc: Add OF init code

irqchip/loongson-pch-lpc: Add OF init code

The OF-based MIPS Loongson-3 systems can also have a PCH LPC interrupt
controller.

Add OF-based initialization code for this driver.

Co-developed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/20260321092032.3502701-6-zhengxingda@iscas.ac.cn