dt-bindings: rcar-gen3-phy-usb2: add no-otg-pins property

This patch adds a new optional property "renesas,no-otg-pins" which
a board does not provide proper otg pins.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>