regulator: Add new driver for ST's PWM controlled voltage regulators

On some STMicroelectronics hardware reside regulators consisting
partly of a PWM input connected to the feedback loop. As the PWM
duty-cycle is varied the output voltage adapts. This driver
allows us to vary the output voltage by adapting the PWM input
duty-cycle.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>