iio: document bindings for mounting matrices

The mounting matrix for sensors was introduced in
commit dfc57732 ("iio:core: mounting matrix support")

However the device tree bindings are very terse and since this is
a widely applicable property, we need a proper binding for it
that the other bindings can reference. This will also be useful
for other operating systems and sensor engineering at large.

I think all 3D sensors should support it, the current situation
is probably that the mounting information is confined in magic
userspace components rather than using the mounting matrix, which
is not good for portability and reuse.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Gregor Boirie <gregor.boirie@parrot.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Samu Onkalo <samu.onkalo@intel.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>