Document the R-Car M3-N (R8A77965) SoC in the R-Car PCIe bindings.

Link: https://lore.kernel.org/r/1604455096-13923-3-git-send-email-yoshihiro.shimoda.uh@renesas.comSigned-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>