hwmon: (pmbus) Add function to clear sensor cache

For PMBus chips, modifying one limit register may affect other limits.
Since limits are all cached in the PMBus core driver, related changes
are not reflected in reported limits.

Introduce function to clear the attribute cache. After calling this function,
the core pmbus driver re-reads all cached values.

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