hwmon: Add support for enable attributes to hwmon core

The hwmon ABI supports enable attributes since commit fb41a710
("hwmon: Document the sensor enable attribute"), but did not
add support for those attributes to the hwmon core. Do that now.

Since the enable attributes are logically the most important attributes,
they are added as first attribute to the attribute list. Move
hwmon_in_enable from last to first place for consistency.

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