misc: Add MAINTAINERS file

misc: Add MAINTAINERS file

This file defines all of the commit keywords used for gem5 commits and
the maintainter(s) for each of these keywords.

This patch introduces a number of new keywords, and changes to previous
keywords. The new keywords better follow gem5's directory structure and
are more extensible.

Currently, most keywords do not have a maintainer. More maintainers will
be added as more people volunteer to be maintainers.

This patch also updates the CONTRIBUTING.md file to point to this file
instead of listing the keywords separately. When this file is committed
the wiki will also be updated accordingly.

Change-Id: Ib0abfeb39a3ca01b74b340e24dc9a2cd95ff813f
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/2760
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>