iio:magnetometer: st_magn: add LSM9DS1 support

Update the sensor settings to support the LSM9DS1 sensor. Although the
LSM9DS1 accelerometer and gyroscope are coupled together to use the same
FIFO, the magnetometer is separate and can be cleanly supported without
refactoring the existing driver.

Signed-off-by: Martin Kelly <martin@martingkelly.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>