Introduce if_mgb driver for Microchip LAN743x PCIe NIC

The Microchip LAN7430 is a PCIe 10/100/1000 Ethernet MAC with integrated
PHY, and the LAN7431 is a MAC with RGMII interface.

To be connected to the build after further testing and review.
Committing now so that changes like r354345 (adding a common
ETHER_IS_ZERO macro) will update this driver too.

Submitted by: Gerald ND Aryeetey <aryeeteygerald_rogers.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20079