dt-bindings: pwm: rockchip: Add description for rk3568

Add "rockchip,rk3568-pwm", "rockchip,rk3328-pwm" compatible strings for
PWM nodes found on a rk3568 platform.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>