dt-bindings: phy: airoha: Add PCIe PHY controller

Introduce device-tree binding documentation for Airoha EN7581 PCIe PHY
controller.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/656133f865433c1d02f00a3abbb1aa9312d2a24e.1718485860.git.lorenzo@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>