rockchip: dts: evb-rk3399: correct pwm3 polarity

The pwm3 on evb-rk3399 is used for pwm regulator, need to invert
the polarity to make it work correctly.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>