Factor out resource "eth_sgmii"

<a href="https://github.com/whitequark"><img src="https://avatars1.githubusercontent.com/u/54771?v=4" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [whitequark](https://github.com/whitequark)**
_Friday Jun 28, 2019 at 03:47 GMT_
_Originally opened as https://github.com/m-labs/nmigen-boards/issues/11_

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Boards using it:
 - [ ] versa_ecp5, versa_ecp5_5g
