regulator: pwm: Adjust PWM config at probe time

The PWM attached to a PWM regulator device might have been previously
configured by the bootloader.
Make sure the bootloader and linux config are in sync, and adjust the PWM
config if that's not the case.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>