iio: imu: st_lsm6dsx: enable sensor-hub support for lsm6dsm

Enabled i2c master controller support for LSM6DSM sensor. Enable
ext_sensor0 for lsm6dsm. This series has been tested using LIS2MDL as
slave device connected to the i2c controller of the LSM6DSM

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>