dt-bindings: PCI: mediatek: Update the Device tree bindings

dt-bindings: PCI: mediatek: Update the Device tree bindings

There are two independent PCIe controllers in MT2712 and MT7622
platform. Each of them should contain an independent MSI domain.

In old dts architecture, MSI domain will be inherited from the root
bridge, and all of the devices will share the same MSI domain.
Hence that, the PCIe devices will not work properly if the irq number
which required is more than 32.

Split the PCIe node for MT2712 and MT7622 platform to comply with
the hardware design and fix MSI issue.

Link: https://lore.kernel.org/r/20210823032800.1660-2-chuanjia.liu@mediatek.com
Signed-off-by: Chuanjia Liu <chuanjia.liu@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh+dt@kernel.org>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>