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

Add internal PCI bridge support for r8a7744 SoC. The Renesas RZ/G1N
(R8A7744) internal PCI bridge is identical to the R-Car Gen2 family.

This doesn't change the driver, so it does nothing by itself.  But it does
mean that checkpatch won't complain about a future patch that adds
"renesas,pci-r8a7744" to a DT, which helps ensure that shipped DTs use
documented compatibility strings.

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>