dt-bindings: phy: rcar-gen2: Add r8a7744 support

Add USB PHY support for r8a7744 SoC. Renesas RZ/G1N (R8A7744)
USB PHY is identical to the R-Car Gen2 family.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>