iio: hmc5843: Add ABI documentation file for hmc5843

Add ABI file documenting hmc5843 non-standard attributes
meas_conf and meas_conf_available for bias current
configuration.
Signed-off-by: Cristina Moraru <cristina.moraru09@gmail.com>
Cc: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-magnetometer-hmc5843

+What:           /sys/bus/iio/devices/iio:deviceX/meas_conf
+What:           /sys/bus/iio/devices/iio:deviceX/meas_conf_available
+KernelVersion:  4.5
+Contact:        linux-iio@vger.kernel.org
+Description:
+                Current configuration and available configurations
+		for the bias current.
+		normal		- Normal measurement configurations (default)
+		positivebias	- Positive bias configuration
+		negativebias	- Negative bias configuration
+		disabled	- Only available on HMC5983. Disables magnetic
+				  sensor and enables temperature sensor.
+		Note: The effect of this configuration may vary
+		according to the device. For exact documentation
+		check the device's datasheet.