docs: arm: convert docs to ReST and rename to *.rst

Converts ARM the text files to ReST, preparing them to be an
architecture book.

The conversion is actually:
  - add blank lines and identation in order to identify paragraphs;
  - fix tables markups;
  - add some lists markups;
  - mark literal blocks;
  - adjust title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by Corentin Labbe <clabbe.montjoie@gmail.com> # For sun4i-ss

arch/arm/common/mcpm_entry.c

@@ -21,7 +21,7 @@
 /*
  * The public API for this code is documented in arch/arm/include/asm/mcpm.h.
  * For a comprehensive description of the main algorithm used here, please
- * see Documentation/arm/cluster-pm-race-avoidance.txt.
+ * see Documentation/arm/cluster-pm-race-avoidance.rst.
  */
 
 struct sync_struct mcpm_sync;