EDAC: Introduce an mci_for_each_dimm() iterator

Introduce an mci_for_each_dimm() iterator. It returns a pointer to
a struct dimm_info. This makes the declaration and use of an index
obsolete and avoids access to internal data of struct mci (direct array
access etc).

 [ bp: push the struct dimm_info *dimm; declaration into the
   CONFIG_EDAC_DEBUG block. ]

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-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20191106093239.25517-4-rrichter@marvell.com