dt-bindings: pwm: renesas-tpu: Document more R-Car Gen2 support

All R-Car Gen2 SoCs have a Renesas Timer Pulse Unit. Document support
for the missing variants.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>