hwmon: epm_adc: Add init/de-init support

Add support to have clients call the de-init of the PSOC
device which disables the EPM GPIO used to power up
the device. When clients call the Init ioctl call the
gpio request to enable the device.

Change-Id: Ifa2e165cb0297728b128aece14e0fea3d314749a
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>