dt-bindings: timer: renesas: cmt: document R-Car gen3 support

Document support for the R-Car gen3 CMT types 0/1 bindings -- they seem
to be the same CMT types 0/1 as in the R-Car gen2 SoCs.

Also document R8A779{7|8}0 bindings as these are the R-Car gen3 SoCs for
which the initial support was done.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>