docs: cma/debugfs.txt: convert docs to ReST and rename to *.rst

The debugfs interface for CMA should be there together with other
mm-related documents.

Convert this small file to ReST and move it to its rightful place.

The conversion is actually quite simple: just add a title for the
document. In order to make it to look better for the audience,
also mark the "echo" command as a literal block.

While this is not part of any book, mark it as :orphan:,
in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>