MAINTAINERS: Add Robert as a EDAC reviewer

I did some significant work with code in edac_mc.c and ghes_edac.c
already, so I guess I can probably help out a bit as code reviewer here.

Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Mauro Carvalho Chehab <mchehab@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/20190902123216.9809-6-rrichter@marvell.com