dt-bindings: timer: renesas, cmt: Document r8a774a1 CMT support

Document SoC specific bindings for RZ/G2M (r8a774a1) SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>