dt-bindings: phy: renesas,rcar-gen3-pcie-phy: Convert to json-schema

Convert the Renesas R-Car Gen3 PCIe PHY Device Tree binding
documentation to json-schema.

Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/0301c3a7dae9afde115b6437a11c23ab85cb8b75.1620648490.git.geert+renesas@glider.be
Signed-off-by: Vinod Koul <vkoul@kernel.org>