dt-bindings: iio: gyroscope: add mount-matrix for bmg160

dt-bindings: iio: gyroscope: add mount-matrix for bmg160

The mount-matrix property supplies a 3x3 matrix that is used
to transform the values from the gyroscope to get vector
values that are relative to the way the sensor has been mounted
on the device. When the property is not specified, the identity
matrix is used.

This change adds mount-matrix as an optional property to the
dt-bindings for the bmg160 gyroscope.

Signed-off-by: Vishwas Rajashekar <vishwas.dev@vrajashkr.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>