Add DT bindings YAML schema for PWM fan controller of LGM SoC

Intel's LGM(Lightning Mountain) SoC contains a PWM fan controller
which is only used to control the fan attached to the system. This
PWM controller does not have any other consumer other than fan.
Add DT bindings documentation for this PWM fan controller.

Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>