[U-Boot] pwm-backlight: make power-supply as option

[Philipp Tomsich]

> Some pwm backlight may not need 'power-supply', let's make it as option
> in pwm-backlight driver.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
>  drivers/video/pwm_backlight.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 

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