dt-bindings: PCI: rcar: Add device tree support for r8a7743

Add support for r8a7743. The Renesas RZ/G1M(R8A7743)PCIe controller
is identical to the R-Car Gen2 family.

No driver change is needed due to the fallback compatible value
"renesas,pcie-rcar-gen2".
Adding the SoC-specific compatible values here has three purposes:
1. Document which SoCs have this hardware module,
2. Allow checkpatch to validate compatible values.
3. Allow the driver to support SoC specific implementations in future
   as necessary.

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