Initial config for minipack3n platform manager

Summary:
** Description **
Add `platform_manager.json` for the minipack3n platform manager.
This configuration has been tested on minipack3.

** Differences **
Due to hardware differences in the SMB section between minipack3 (mp3) and minipack3n (mp3n),
platform_manager logs are expected to show errors related to `SMB_VRM*`.

** Test Plan **
- Run the platform_manager with this configuration on minipack3n.
- Validate the generated logs to ensure functionality, excluding the known SMB-related errors.

[minipack3n-platform_manager_execute_on_mp3_log_20241212.txt](https://github.com/user-attachments/files/18193421/minipack3n-platform_manager_execute_on_mp3_log_20241212.txt)

Author: brandonchuang