(gpio-fan): Add thermal control hooks

Allow gpio-fan to be used as thermal cooling device for platforms that
use GPIO maps to control fans.

As part of this change, we make the shutdown and remove logic the same
as well.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>