hwmon: epm_adc: Add average buffering command

The average buffering is used by clients who want
to set a period for which they want the PSoC to
measure and collect samples for the enabled channels.
The data obtained from the PSoC is a 12bit data for
each channel measurement. The EPM driver parses the
data for each of this channel and stores the result
for clients who issued the average buffered request.

Change-Id: I78dd7024c3146aa9578dc65e497021e42efbbd45
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>