Add driver for Microchip LAN78xx USB3-GigE controller

This driver supports two Microchip USB-Ethernet controllers:

LAN7800 USB 3.1 to 10/100/1000 Mbps Ethernet
LAN7515 USB 2 to 10/100/1000 Mbps Ethernet with built-in USB hub

The LAN7515 is the Ethernet controller on the Raspberry Pi 3B+.
At present there is no datasheet for the LAN7515, but it is effectively
a USB 2 hub combined with a LAN7800 controller. A comprehensive LAN7800
datasheet is at http://www.microchip.com/wwwproducts/en/LAN7800.

This driver is based on the structure of the smsc(4) driver which
supports Microchip/SMSC's LAN95xx family. (Microchip acquired SMSC
in May 2012.) The Linux lan78xx driver served as a reference for some
functionality and registers.

The 'muge' driver name comes from "Microchip USB Gigabit Ethernet".
I made some style adjustments and minor edits to Arshan's submission.
It will be connected to the build after additional review and testing.

Thanks to Microchip for providing a number of Evaluation Boards (EVBs)
for development and testing.

Submitted by: Arshan Khanifar
Reviewed by: hselasky (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15168