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>