ARM: dts: Add main PWM info to rk3288

This adds the PWM info (other than the VOP PWM) to the main rk3288
dtsi file.

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