MAINTAINERS: update SGI & ia64 Altix stuff

Dimitri and Robin have taken over GRU maintenance.

Linux on Altix is no longer maintained except as part of ia64, and
there's already a separate IA64 maintainer entry.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>