sfxge: add medford_impl.h, medford_nic.c, ef10_impl.h

Creating some files together to do the build system changes in one go.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4859