hwmon: Add QTI AMOLED ECM driver

AMOLED ECM (Embedded Current Measurement) is a method to measure OLED
display power consumption with a time granularity in sub-frame or
multiple frames of image data. ECM can be for a short period or for a
longer period.

Change-Id: I7ea0b7d8d5961ec015c5a79ee2037ff36329f775
Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>