hwmon: (adm1275) Allow setting sample averaging

Current driver assume PWR_AVG and VI_AVG as 1 by default, and user needs
to set sample averaging via sysfs manually.

This patch parses the properties "adi,power-sample-average" and
"adi,volt-curr-sample-average" from device tree, and setting sample
averaging during probe. Input value must be one of value in the
list [1, 2, 4, 8, 16, 32, 64, 128].

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Link: https://lore.kernel.org/r/20220302123817.27025-2-potin.lai@quantatw.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>