dt-bindings: PCI: Add support for MT7629

MT7629 is an ARM based platform SoC integrating the same PCIe IP as
MT7622, add a binding for it.

Signed-off-by: Jianjun Wang <jianjun.wang@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>