ARM: dts: rockchip: fix PWM clock found on RK3288 Socs

We use the new PWM IP on RK3288, but the PWM's clock indeed incorrect.

Signed-off-by: Caesar Wang <caesar.wang@rock-chips.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>