dt-bindings: iio: imu: st_lsm6dsx: add lsm9ds1 device bindings

Note the binding is st,lsm9ds1-imu.  This device is effectively
two separate devices in the same package. (separate addreses,
chip selects etc) The magnetometer is already supported
as lsm9ds1-mag.  imu may not be the best name but it's the best
that anyone has yet come up with for a gyro and accel part.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>