dt-bindings: pci: rcar-pci-ep: Document r8a774a1 and r8a774b1

dt-bindings: pci: rcar-pci-ep: Document r8a774a1 and r8a774b1

Document the support for R-Car PCIe EP on R8A774A1 and R8A774B1 SoC
devices.

Also constify "renesas,rcar-gen3-pcie-ep" so that it can be used as
fallback compatible string for R-Car Gen3 and RZ/G2 devices as the
PCIe module is identical.

Link: https://lore.kernel.org/r/20200814173037.17822-2-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>