PCI: rcar: Add device tree support for r8a7743/5

Add internal PCI bridge support for r8a7743/5 SoC.  The Renesas RZ/G1[ME]
(R8A7743/5) 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-r8a7743" to a DT, which helps ensure that shipped DTs use
documented compatibility strings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
[bhelgaas: add explanatory note]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>