dt-bindings: PCI: Add documentation for MediaTek PCIe

Add documentation for PCIe host driver available in MT7623 series SoCs.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>