dt-bindings: phy: renesas: usb3-phy: convert bindings to json-schema

Convert Renesas R-Car generation 3 USB 3.0 PHY bindings documentation
to json-schema.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1585301636-24399-4-git-send-email-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>