iio:gyro:bmg160 documentation

Added any-motion trigger documentation.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-gyro-bmg160

+What:		/sys/bus/iio/devices/triggerX/name = "bmg160-any-motion-devX"
+KernelVersion:	3.17
+Contact:	linux-iio@vger.kernel.org
+Description:
+		The BMG160 gyro kernel module provides an additional trigger,
+		which sets driver in a mode, where data is pushed to the buffer
+		only when there is any motion.