uboot-imx

pwm-backlight: make power-supply as option

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@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>