if_muge: connect module to the build

Supported Microchip devices:

• LAN7515 USB 2 hub and gigabit Ethernet controller w/ PHY
• LAN7800 USB 3.0 gigabit Ethernet controller w/ PHY

The LAN7515 is the Ethernet controller on the Raspberry Pi 3 B+.

This driver has some TODO items outstanding, but basic functionality
works.

Sponsored by: The FreeBSD Foundation