dt-bindings: timer: renesas, cmt: Document r8a774b1 CMT support

Document SoC specific bindings for RZ/G2N (r8a774b1) SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1570104229-59144-1-git-send-email-biju.das@bp.renesas.com