top: Added Etherbone support.

- Added Etherbone core as wishbone master in top crossbar.
- Added a multiplexer to connect Etherbone and Nic core to wr-core.
- Connected wr-core to Etherbone configuration space to access Etherbone registers.
- wrc.ram: Updated with Etherbone support. (Old: wrc_without_etherbone.ram)
- wrc.ram: Updated ep_pfilter for Etherbone packets. You can see this configuration in extra folder.