regulator: pwm-regulator: Re-write bindings

* Add support for continuous-voltage mode
* Put more meat on the bones with regards to voltage-table mode
* Sort out formatting for ease of consumption

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