hwmon: epm_adc: Add configurable global GPIO enable

Embedded power monitor (EPM) PSoC is powered through a GPIO.
Add a property to pass the GPIO to the driver since this
the global enable for the EPM varies from target to target.

Change-Id: I731302f9ab33c216f5794721a4d36ee20fb261ec
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>