dt-bindings: timer: renesas,cmt: Document r8a7742 CMT support

Document SoC specific compatible strings for r8a7742. No driver change
is needed as the fallback strings will activate the right code.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@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/20200902091927.32211-1-prabhakar.mahadev-lad.rj@bp.renesas.com