dt-bindings: phy: mediatek: Add YAML schema for PCIe PHY

Add YAML schema documentation for PCIe PHY on MediaTek chipsets.

Signed-off-by: Jianjun Wang <jianjun.wang@mediatek.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220617070246.20142-2-jianjun.wang@mediatek.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>