Add INTx support for MediaTek Gen3 PCIe controller.

Link: https://lore.kernel.org/r/20210420061723.989-5-jianjun.wang@mediatek.comSigned-off-by: Jianjun Wang <jianjun.wang@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>