dt-bindings: timer: renesas, cmt: Add CMT0 and CMT1 to r8a7792

This patch adds DT binding documentation for the CMT devices on
the R-Car Gen2 V2H (r8a7792) SoC.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>