ARM: tegra: Add dummy backlight power supplies

The Medcom Wide and PAZ00 boards don't specify the power supply for the
backlight, which means that the Linux driver will provide a dummy one.
Wire up an explicit dummy to also make the DT schema validation succeed.

Unfortunately I don't have access to the schematics for the Medcom Wide,
so I don't know if a more accurate description is possible.

The AC100 (PAZ00) schematics from here:

	https://www.s-manuals.com/pdf/motherboard/compal/compal_la-6352p_r1.0a_schematics.pdf

aren't entirely clear which one of the supplies powers backlight, but
the panel supply is probably close enough.

Based on work by David Heidelberg <david@ixit.cz>.
Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arm/boot/dts/tegra20-medcom-wide.dts

@@ -54,6 +54,9 @@ backlight: backlight {
 
 		brightness-levels = <0 4 8 16 32 64 128 255>;
 		default-brightness-level = <6>;
+
+		/* close enough */
+		power-supply = <&vdd_3v3_reg>;
 	};
 
 	panel: panel {

arch/arm/boot/dts/tegra20-paz00.dts

@@ -571,6 +571,9 @@ backlight: backlight {
 
 		brightness-levels = <0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 255>;
 		default-brightness-level = <10>;
+
+		/* close enough */
+		power-supply = <&vdd_pnl_reg>;
 	};
 
 	clk32k_in: clock-32k {