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>

Documentation/cma/debugfs.txt → Documentation/cma/debugfs.rst

+:orphan:
+
+=====================
+CMA Debugfs Interface
+=====================
+
 The CMA debugfs interface is useful to retrieve basic information out of the
 different CMA areas and to test allocation/release in each of the areas.
 
@@ -12,7 +18,7 @@ The structure of the files created under that directory is as follows:
  - [RO] count: Amount of memory in the CMA area.
  - [RO] order_per_bit: Order of pages represented by one bit.
  - [RO] bitmap: The bitmap of page states in the zone.
- - [WO] alloc: Allocate N pages from that CMA area. For example:
+ - [WO] alloc: Allocate N pages from that CMA area. For example::
 
 	echo 5 > <debugfs>/cma/cma-2/alloc