igc(4): Introduce new driver for the Intel I225 Ethernet controller.

This controller supports 2.5G/1G/100MB/10MB speeds, and allows
tx/rx checksum offload, TSO, LRO, and multi-queue operation.

The driver was derived from code contributed by Intel, and modified
by Netgate to fit into the iflib framework.

Thanks to Mike Karels for testing and feedback on the driver.

Reviewed by: bcr (manpages), kbowling, scottl, erj
Relnotes: yes
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30668

For stable/12, CTLFLAG_NEEDGIANT removed and if_foreach_llmaddr() replaced with
if_multiaddr_array().

(cherry picked from commit 517904de5ccac643589c71ac0d2751797f89e4f9)