dt-bindings:iio:magnetometer:asahi-kasei,ak8974: txt to yaml format conversion

Simple conversion.  I have pruned descriptions that did not add much useful
detail.  Note that the mount-matrix description will form part of a generic
IIO binding.  No need to repeat that in every driver that uses it.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031184854.745828-25-jic23@kernel.org