regulator: pwm: Document pwm-dutycycle-unit and pwm-dutycycle-range

Document the pwm-dutycycle-unit and pwm-dutycycle-range properties.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>