iio: core: Introduce IIO_ACTIVITY channel

This channel will be used for exposing information about
activity composite sensors. Activities supported so far:
	* running
	* jogging
	* walking
	* still

THRESHOLD event is used to signal a change in the activity
state.

We associate a confidence interval for each activity expressed
as a percentage from 0 to 100.
  * 0, means the sensor IS NOT reporting that activity.
  * 100, means the sensor IS reporting that activity.

Users of this interface have two possible means to gather
information about the ongoing activities.

1. Event based, via event file descriptor
  * sensor may report an event when ENTERING an activity or LEAVING
    an activity based on a threshold value.
  * drivers will wake up applications waiting data on the event fd

2. Polling, by reading the sysfs associated attribute files:
  * /sys/bus/iio/devices/iio:device0/in_activity_running_input
expressed as percentage confidence value from 0 to 100.

This will offer an interface for Android significant motion
composite sensor defined here:
http://source.android.com/devices/sensors/composite_sensors.html

Activities listed above are supported by Freescale's MMA9553 sensor:
http://freescale.com/files/sensors/doc/ref_manual/MMA9553LSWRM.pdf

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/types.h

@@ -30,6 +30,7 @@ enum iio_chan_type {
 	IIO_CCT,
 	IIO_PRESSURE,
 	IIO_HUMIDITYRELATIVE,
+	IIO_ACTIVITY,
 };
 
 enum iio_modifier {
@@ -59,7 +60,11 @@ enum iio_modifier {
 	IIO_MOD_NORTH_MAGN,
 	IIO_MOD_NORTH_TRUE,
 	IIO_MOD_NORTH_MAGN_TILT_COMP,
-	IIO_MOD_NORTH_TRUE_TILT_COMP
+	IIO_MOD_NORTH_TRUE_TILT_COMP,
+	IIO_MOD_RUNNING,
+	IIO_MOD_JOGGING,
+	IIO_MOD_WALKING,
+	IIO_MOD_STILL,
 };
 
 enum iio_event_type {