dt-bindings: timer: renesas, cmt: Document r8a77470 CMT support

Document SoC specific compatible strings for r8a77470. No driver change
is needed as the fallback strings will activate the right code.

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