dt-bindings: PCI: mediatek-gen3: Add YAML schema

Add YAML schemas documentation for Gen3 PCIe controller on
MediaTek SoCs.

Link: https://lore.kernel.org/r/20210420061723.989-2-jianjun.wang@mediatek.com


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>