mgb: Connect if_mgb module to the build

It supports the following Microchip devices:

LAN7430 PCIe Gigabit Ethernet controller with PHY
LAN7431 PCIe Gigabit Ethernet controller with RGMII interface

The driver has a number of caveats and limitations, but is functional.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 543df609072fe49079c36d6bee510e1645edde3a)