samples: boards: mimxrt595_evk_cm33: system_off: misc cleanups

- Remove unnecessary includes
- Remove redundant definitions (sample is not portable anyway)
- Use printf
- Check for device readiness
- Do not check for zero ticks (not possible for the given
  timeout/driver), also, driver should likely error for such case anyway
- Check for all error codes

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>