dt-bindings: timer: renesas, cmt: Document r8a774[35] CMT support

Document SoC specific compatible strings for r8a7743 and r8a7745.
No driver change is needed as the fallback strings will activate
the right code.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>