mt76: add mt7615 to the pile of buildable drivers

Ignoring page_pools with the few needed adjustments and ignoring 7622
mt7615 seems to build as well. Add it so once we can connect it to
the build people can start testing and debugging.
(The actual work was done on a newer version of the mt76 drivers but
it seems the to-build-changes equally apply here already).

Requested by: Radu-Cristian Fotescu (freebsd-wireless, 2024-07-31)
Sponsored by: The FreeBSD Foundation
MFC after: 3 days