iio: meter: ina3221: add FORCED_TRIGGERED mode

iio: meter: ina3221: add FORCED_TRIGGERED mode

This patchs adds an explicit triggered mode into ina3221 driver. There
are now four modes of operation for ina3221 in the device driver
 (1) FORCED_TRIGGERED
 (2) FORCED_CONTINUOUS
 (3) TRIGGERED
 (4) CONTINUOUS

First two (1, 2) are selectable from user space. If mode is not forced
from user space, and critical or warning current limit has been
configured, driver automatically picks the mode from latter two (3, 4)
depending on the system activity.

Operation mode of ina3221 can be selected by writing into running_mode
sysfs entry.
 * Writing >0 enables FORCED_CONTINUOUS mode
 * Writing 0 enables FORCED_TRIGGERED mode
 * Writing <0 enables automatic mode selection of the driver

Change-Id: I935e9be3280b46b5ee3dd83f50e4845ae2eafe0b
Signed-off-by: Timo Alho <talho@nvidia.com>
Reviewed-on: http://git-master/r/392886
(cherry picked from commit 017a1d491ccd08a6a49cdf4f5791654ba8f49c12)
Reviewed-on: http://git-master/r/395106
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Steve Rogers <srogers@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Juha Tukkinen <jtukkinen@nvidia.com>