EDAC/Documentation: Describe CPER module definition and DIMM ranks

Update on CPER DIMM naming convention and DIMM ranks.

 [ bp: Touchups. ]

Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>
Cc: "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20191106093239.25517-14-rrichter@marvell.com