util: Use MAINTAINERS.yaml for valid tags in git hook

util: Use MAINTAINERS.yaml for valid tags in git hook

There is a mismatch between the tags in MAINTAINERS.yaml and the
valid_tags in the git hook. This means if a user consults the
MAINTAINERS.yaml file to find the appropriate tag, there is a chance of
the commit being rejected due to this mismatch. Now that the maintainers
file is in yaml format, use the util/maint library to parse the valid
tag options. Additional meta tags are added (WIP, RFC) and tags that
were previously valid but not in the MAINTAINERS.yaml file.

Change-Id: I3de8f0b6f8507aa1afd2118bc4373ac0610cce40
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37220
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>