hwmon: (pmbus) Add more virtual registers

Add PMBUS_VIRT_READ_TEMP_AVG, PMBUS_VIRT_READ_TEMP2_AVG,
PMBUS_VIRT_READ_POUT_AVG, PMBUS_VIRT_READ_POUT_MAX,
and PMBUS_VIRT_RESET_POUT_HISTORY.

We'll need those for MAX34446.

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