dt-bindings: phy: renesas,rcar-gen2-usb-phy: Convert to json-schema

Convert the Renesas R-Car Gen2 USB PHY Device Tree binding documentation
to json-schema.

Add missing properties.
Rename the device node from "usb-phy" to "usb-phy-controller", as it
does not represent a USB PHY itself, and thus does not have a
"#phy-cells" property.
Rename the child nodes from "usb-channel" to "usb-phy", as these do
represent USB PHYs.
Drop the second example, as it doesn't add any value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/dbdcffd009302734fe2fb895ce04b72fa1ea4355.1663165000.git.geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>