dt-bindings: phy: add binding for the Lantiq VRX200 and ARX300 PCIe PHYs

Add the bindings for the PCIe PHY on Lantiq VRX200 and ARX300 SoCs.
The IP block contains settings for the PHY and a PLL.
The PLL mode is configurable through a dedicated #phy-cell in .dts.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>