dt-bindings: iio: imu: st,lsm6dsx: add mount-matrix property

Add the mount-matrix optional property to the binding since it's supported
and very useful when using the chip on a board.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231012142041.253332-1-martink@posteo.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>