Add support for r8a77961 (R-Car M3-W+).

To avoid confusion between R-Car M3-W (R8A77960) and R-Car M3-W+
(R8A77961), this patch also updates the comment of
"renesas,pcie-r8a7796".

Link: https://lore.kernel.org/r/1586511020-31833-1-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>