Document the support for R-Car PCIe EP on R8A774E1 SoC device.

Link: https://lore.kernel.org/r/20200904103851.3946-2-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-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>