dt-bindings: timer: renesas,cmt: Document r8a774e1 CMT support

Document SoC specific bindings for RZ/G2H (r8a774e1) SoC.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@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/1594811350-14066-6-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com