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/mips/bmips/setup.c

@@ -162,7 +162,7 @@ void __init plat_mem_setup(void)
 	ioport_resource.start = 0;
 	ioport_resource.end = ~0;
 
-	/* intended to somewhat resemble ARM; see Documentation/arm/Booting */
+	/* intended to somewhat resemble ARM; see Documentation/arm/booting.rst */
 	if (fw_arg0 == 0 && fw_arg1 == 0xffffffff)
 		dtb = phys_to_virt(fw_arg2);
 	else if (fw_passed_dtb) /* UHI interface or appended dtb */