rockchip: dts: rk3128: update pwm-cell for pwm0

The backlight pwm-cell is 3.

This remove the warning in buildman:
arch/arm/dts/rk3126-evb.dtb: Warning (pwms_property): Property 'pwms', cell 3 is not a phandle reference in /backlight
arch/arm/dts/rk3126-evb.dtb: Warning (pwms_property): Missing property '#pwm-cells' in node /sram@10080400 or bad phandle (referred from /backlight:pwms[3])

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>