rockchip: board: evb_rk3399: initialize pwm0 for dispaly backlight

Enable pwm0 for display of rk3399 evb board. The PWM do not have decicated
interrupt number in dts and can not get periph_id by pinctrl framework. So
init them here.

Signed-off-by: Eric Gao <eric.gao@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>