hwmon: (dell-smm) Add support for fanX_min, fanX_max and fanX_target

The nominal speed of each fan can be obtained with
i8k_get_fan_nominal_speed(), however the result is not available
from userspace.
Change that by adding fanX_min, fanX_max and fanX_target attributes.
All are RO since fan control happens over pwm.

Tested on a Dell Inspiron 3505 and a Dell Latitude C600.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210926221044.14327-2-W_Armin@gmx.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>