iio: magn: DT bindings for AK8974

This adds device tree bindings for the AK8974 magnetometer, similar
to those for the AK8975.

Cc: devicetree@vger.kernel.org
Cc: Samu Onkalo <samu.onkalo@intel.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>