regulator: pwm-regulator: Add support for continuous-voltage

The current version of PWM regulator only supports a static table
approach, where pre-calculated values are supplied by the vendor and
obtained via DT.  The continuous-voltage method takes min_uV and
max_uV, and divides the difference between them up into a number of
slices.  The number of slices depend on how large the duty cycle
register is.  This information is provided by a DT property.

As the name alludes, this provides values for a continuous voltage
range between min_uV and max_uV, which has obvious benefits over
either limited voltage possibilities, or the requirement to provide
a large voltage-table.

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