hwmon: (lm73) Add 'update_interval' attribute

The LM73 supports four A/D conversion resolutions.  The default used by
the existing lm73 driver is the chip's default, 11-bit (0.25 C/LSB).
This patch enables changing of this resolution from userspace via the
update_interval sysfs attribute.  Full details on usage are included in
Documentation/hwmon/lm73.

Signed-off-by: Chris Verges <kg4ysn@gmail.com>
[linux@roeck-us.net: cleanup]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>