dt-bindings: interrupt-controller: Add LS7A PCH LPC

dt-bindings: interrupt-controller: Add LS7A PCH LPC

Loongson 7A series PCH contains an LPC controller with an interrupt
controller.

Add the device tree binding for the interrupt controller.

Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260321092032.3502701-4-zhengxingda@iscas.ac.cn