s3c_adc_battery: Average over more than one adc sample

Some sources for adc battery information provide only inaccurate results
where the read value differs from the real value with positive and negative
offsets. For such sources it can be more accurate to collect two or more
value sample and use the average 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>