dt-bindings: PCI: rcar: Add device tree support for r8a774a1

Add PCIe support for the RZ/G2M (a.k.a. R8A774A1).

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Simon Horman <horms+renesas@verge.net.au>