RAS: Add a MAINTAINERS entry

This was long overdue and it is needed at least so that Tony and I get
CCed on patches.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Tony Luck <tony.luck@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20190125174227.10652-1-bp@alien8.de