regulator: Add driver for voltage controlled regulators

The output voltage of a voltage controlled regulator can be controlled
through the voltage of another regulator. The current version of this
driver assumes that the output voltage is a linear function of the control
voltage.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>