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>