hwmon: (lm25066) Support SAMPLES_FOR_AVG register

Manufacturer specific SAMPLES_FOR_AVG register allows setting the number
of samples used in computing the average values (PMBUS_VIRT_READ_*_AVG).
The number we write is an exponent of base 2 of the number of samples so
for example writing 3 will result in 8 samples average.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>