hwmon: (max31790) add fanN_enable

The MAX31790 has a tach input enable bit in each fan's configuration
register. This is only enabled by the driver if RPM mode is selected,
but the driver doesn't provide a way to independently enable tachometer
input regardless of the regulator mode.

By adding the fanN_enable sysfs files, we can decouple the tach input
from the regulator mode. Also update the documentation.

Signed-off-by: Justin Ledford <justinledford@google.com>
Link: https://lore.kernel.org/r/20220829195930.2521755-1-justinledford@google.com


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