litescope/bridge: create a generic wishbone bridge that can be used with different...

litescope/bridge: create a generic wishbone bridge that can be used with different phys (the phy needs to provide a sink/source with 8bits data).

- we can now pass a phy to LiteScopeWishboneBridge
- LiteScopeUART2Wishbone is only a specific LiteScopeWishboneBridge
- UART mux is removed since complicated and no longer useful (we can now create easily virtual UART over Ethernet, USB or PCIe) or simply add another UART for debug.