dt-bindings: timer: renesas, cmt: Document r8a779{5|65|90} CMT support

Document SoC specific bindings for R-Car H3/M3-N/E3 SoCs.

Signed-off-by: Cao Van Dong <cv-dong@jinso.co.jp>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>