dt-bindings: phy: rcar-gen3-phy-usb2: Revise #phy-cells property

To have the detailed property on each PHY specifier, this patch revises
the #phy-cells property.

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