hwmon: epm_adc: Add Marker support

Clients can request start/stop markers and perform
current measurement. Support is added to request a
marker and set a desired level. When client
has finished it needs to call release for freeing
the gpio.

Change-Id: I3e805352a8c4cd56e616f76fc77e8fb7fa97fb50
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>