MAINTAINERS: add initial entries for west.yml projects

This is part of the road towards replacing CODEOWNERS with
MAINTAINERS (tracked in the process working group as #38725).

As part of that work, we wanted to have a way to track the maintainers
of each west project (entry in west.yml) by name of project.

This is an initial attempt at that, based mostly on my personal
knowledge, the git logs, and some rough guesswork. I fully expect
omissions and errors, but it should be enough to get us started, with
fixes to follow incrementally from the people who know their
individual areas better than I do.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>