Add genet driver for Raspberry Pi 4B Ethernet

Add driver for Broadcom "GENET" version 5, as found in BCM-2711 on
Raspberry Pi 4B. The driver is derived in part from the bcmgenet.c
driver in NetBSD, along with bcmgenetreg.h.

Reviewed by: manu
Obtained from: in part from NetBSD
Relnotes: yes, note addition
Differential Revision: https://reviews.freebsd.org/D24436