hwmon: (pmbus) Add pmbus_set_update() function to set update flag

hwmon: (pmbus) Add pmbus_set_update() function to set update flag

For the STPDDC60 chip, the vout alarm-limits are represented as an offset
relative to the commanded output voltage. This means that the limits are
dynamic and must not be cached by the pmbus driver. This patch adds a
pmbus_set_sensor() function to pmbus_core to be able to set the update flag
on selected sensors after auto-detection of limit attributes.

Signed-off-by: Erik Rosen <erik.rosen@metormote.com>
Link: https://lore.kernel.org/r/20210218115249.28513-2-erik.rosen@metormote.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>