pwm: Add PWM fan controller driver for LGM SoC

Intel Lightning Mountain(LGM) SoC contains a PWM fan controller. This
PWM controller does not have any other consumer, it is a dedicated PWM
controller for fan attached to the system. Add driver for this PWM fan
controller.

Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>