iio: magnetometer: st_magn: Provide default platform data

Provide default platform data for magnetometer in case it supports DRDY.

One case is LSM9DS0 IMU, on which it is the case. Since accelerometer
is using INT1, default magnetometer to INT2.

While at it, update description of the drdy_int_pin field.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210414195454.84183-3-andriy.shevchenko@linux.intel.com


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