new(mlslcli): test psb reg read and interrupt

Test interrupt status while reading PS board register.

DISCLAIMER: This git commit has really messy git log (this was branched from compat-old-regmap, but compat-old-regmap branch was rebased (in kind of a dirty way) so the commits are not the same). The reason why I don't also rebase this branch is that the rebase breaks the branch (as it did on compat-old-regmap). I used rebase and squashing on this repo for merging, but it is actually harder scheme than just making merge commits. Anyway, managing two different branches in parallel and having too large diffs are really difficult.

TODO: cherry-pick this on main