[U-Boot] rockchip: dts: rk3128: update pwm-cell for pwm0

[Philipp Tomsich]

> 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 at 10080400 or bad phandle (referred from /backlight:pwms[3])
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
>  arch/arm/dts/rk3128.dtsi | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 

Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>