samples: sensor: Extend magn_polling to support additional drivers

Extends the magn_polling sample application to support additional
magnetometer drivers. This is a first step towards reducing the number
of driver-specific sensor sample applications.

Integration platforms were selected to exercise different magnetometer
drivers.

Tested on hardware with the following configurations:
- fxos8700 driver with the on-board sensor on the frdm_k64f board
- lis2mdl driver with a x_nucleo_iks01a3 shield attached to a frdm_k64f
  board

Signed-off-by: Maureen Helm <maureen.helm@intel.com>