dt-bindings: phy: rcar-gen2: Add r8a7742 support

Add USB PHY support for r8a7742 SoC. Renesas RZ/G1H (R8A7742)
USB PHY is identical to the R-Car Gen2 family.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>