soc: arm: atmel_sam0: improve radio off handling for samr3x

Improve the radio off code, mainly:

- Compile the file only if necessary, ie, LORA radio not in use
- Use pin information from DT, so that we do not need to hardcode pins
  and can switch to dt-spec APIs.
- Improve error handling, includes, etc.

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