dt-bindings: pwm: tpu: Add R-Car V3U device tree bindings

Add device tree bindings for TPU with the PWM controller found
on R-Car V3U SoCs.

Signed-off-by: Duc Nguyen <duc.nguyen.ub@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>