dt-bindings: pwm: rockchip: Add description for rk3588

Add "rockchip,rk3588-pwm" compatible string for PWM nodes found
on a rk3588 platform.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>