ge of all collected values.

This patch adds pdata options volt_samples, current_samples and
backup_volt_samples to specifiy the number of samples to collect,
reads the specified number of samples and calculates the average of those.
For unset sample-number-values a default of 1 is assumed.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
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