Add genet.4 for RPi 4 Ethernet

Add new man page for genet(4) Ethernet on Raspberry Pi 4B, based on
several other Ethernet man pages. Hook into build.

Note, this could potentially be added as an aarch64 man page; not
sure if that matters now. Include if_genet(4) link as for other
network devices.

Copyright notice cloned from a recent FreeBSD Foundation copyright.

MFC after: 3 days
Reviewed by: imp bcr manpages
Differential Revision: https://reviews.freebsd.org/D33360