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The DCDC6 power rail is typically driving VDD_SYS in the SoC, so it is
on by default and uses the default voltage.

As there seems to be at least on board using a different voltage, add
the rail to the list of known voltage lines, so we can setup the right
voltage as early as possible.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>