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The following warning is observed on Jetson TX1 platform because the
supply regulator is not specified for the backlight.

 WARNING KERN lp855x 0-002c: 0-002c supply power not found, using dummy regulator

The backlight supply is provided by the 3.3V SYS rail and so add this
as the supply for the backlight.
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>