hwmon: (core) Add hwmon_in_enable attribute

According to hwmon ABI, in%d_enable is a sysfs interface that
allows user space to enable and disable the input sensor. So
this patch just simply adds the attribute to the list.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>