dt-bindings: PCI: rcar: Add device tree support for r8a7744

Add support for r8a7744. The Renesas RZ/G1N (R8A7744) PCIe controller
is identical to the R-Car Gen2 family.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>