driver: ethernet: Energy Detect Power-Down feature for Microchip enc424j600

[wniva]

Problem: High power consumption when there is no Ethernet connection.

Solution: Implement the Power-Saving Feature "Energy Detect Power-Down"

Alternatives: Not available

Additional context:
See datasheet "16.2 Power Detect Shutdown"
-> "To enable Power Detect Shutdown mode, set the EDPWRDN bit (PHCON2<13>)."

[maass-hamburg]

@wniva feel free zo submit a PR to fix it.