devicetree: bindings: R-Car Gen2 CMT0 and CMT1 bindings

Add documentation for new separate CMT0 and CMT1 DT compatible strings
for R-Car Gen2. These compat strings allow us to enable CMT1-specific
features in the driver. The old compat strings will be deprecated in
the not so distant future.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>