dt-bindings: hwmon: Add optional regulator support to pwm-fan

This adds an optional regulator support (e.g. switchable supply) to the
pwm fan binding.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>