samples: drivers: ps2: fix test issues

- Test effectively requires a microchip,xec-ps2, but it was "optional"
  even though the test continued setting up a callback that makes use of
  the device.
- Improve test filtering so that it requires a valid compatible to be
  enabled (allows to delete redundant ps2 tag)
- Initialize device at compile time (allowing to constify its pointer)

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>