arm64: tegra: Populate LP8557 backlight regulator

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>

arch/arm64/boot/dts/nvidia/tegra210-p2371-2180.dts

@@ -56,6 +56,7 @@ i2c@7000c400 {
 		backlight: backlight@2c {
 			compatible = "ti,lp8557";
 			reg = <0x2c>;
+			power-supply = <&vdd_3v3_sys>;
 
 			dev-ctrl = /bits/ 8 <0x80>;
 			init-brt = /bits/ 8 <0xff>;