iio: imu: st_lsm6dsx: introduce ST_LSM6DSX_ID_EXT sensor ids

Add ST_LSM6DSX_ID_EXT{0,1,2} sensor ids as reference for slave devices
connected to st_lsm6dsx i2c controller. Moreover introduce odr dependency
between accel and ext devices since i2c embedded controller relies on the
accelerometer sensor as bus read/write trigger so we need to enable accel
device at odr = max(accel_odr, ext_odr) in order to properly communicate
with i2c slave devices

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