hwmon: (ds1621) Add ds1721 update interval sysfs attribute

The ds1721 device can be configured for 9..12 bit resolutions;
add a sysfs attribute for userspace to configure this attribute.
The definition, description, details, and usage are shown in the
documentation and were crafted from an LM73 driver patch done by
Chris Verges & Guenter Roeck).

Signed-off-by: Robert Coulson <rob.coulson@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>