drivers: ethernet: eth_sam_gmac: Detect and report link status

This commit adds the "monitor task" to detect and report any changes
in the PHY link status to the operating system.

The monitor task is perodically executed to poll the link status from
the PHY and call `net_eth_carrier_{off,on}` based on the detected
link status change.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>