Many voltage Regulators need a input voltage that is higher than the
output voltage. Allow to specify a minimum dropout voltage which will
be used later to find the best input voltage for regulators.

[Changed uv to uV for consistency and legibility -- broonie]
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NMark Brown <broonie@kernel.org>