dt-bindings: PCI: rcar: Add device tree support for r8a774b1

Add PCIe support for the RZ/G2N (a.k.a. R8A774B1).

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>