hwmon: epm_adc: Add 16 bit buffering and generic support

Add support to return 16bit results for averaging buffering
command. This allows additional channels to be measured.
The generic command allows clients to send requests that
are supported by the PSoC. In addition fix the marker
gpio numbers between the marker 1 and marker 2 that are
flipped.

Change-Id: I2bf87ceb8fa9fa633ab6d082b12f66c77a374d34
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>