lindebugfs: implement debugfs_create_devm_seqfile()

[bz]

Summary

Implement debugfs_create_devm_seqfile() keyed by device and a
read function.

XXX I am not really sure where to put all the bits as they do not
seem to really belong in lindebugfs, but also do not really fit
into debugfs.h (rather fs.h?)
I have thus left over a few bits to cleanup in the hope we can
sort the belonings of the implementation out first.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

[bz]

Please see the XXX in the proposed commit message for now. I still wonder which headers and implementation files to put these things given they are this weird mix between device binding and debugfs.

I am expecting to move things around as a result so technical or style review maybe after this is sorted and I updated the review.