pwm: Add support for sl28cpld PWM controller

Add support for the PWM controller of the sl28cpld board management
controller. This is part of a multi-function device driver.

The controller has one PWM channel and can just generate four distinct
frequencies.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>