iio: imu: st_lsm6dsx: add 'mount_matrix' sysfs entry to gyro channel.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/1673874434-30750-3-git-send-email-Philippe.DeMuyter@macq.eu


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