dt-bindings: pwm: renesas,tpu: Add r8a779g0 support

Document support for the 16-Bit Timer Pulse Unit (TPU) in the Renesas
R-Car V4H (R8A779G0) SoC.

Based on a patch in the BSP by CongDang.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>