[netX51] Introduce software timer for Link Change handling in FiberOptic Mode

Due to a known issue in netX51 chip in case of FiberOptic Ethernet mode, the Link Change handling needs to be changed. However, the change shall only be used for

• netX51/52 chip
• if at least one Ethernet port is configured for FiberOptic Mode

The following change shall be implemented:

• On startup one software timer per port shall be created (300ms timeout)
• After a Link Change, check timer status
  • if timer is not running, immediately handle the Link Change event and start the timer
  • if timer is running, no action is done
• Once the timer expires, the Link Change shall be checked and handled.

Currently each Link Change is handled immediately.