MAINTAINERS: Change ownership for SGI specific modules.

I have taken a different job.  I am removing myself as maintainer of
GRU.  Dimitri will continue to maintain the SGI GRU driver, changing the
XP/XPC/XPNET maintainer to Cliff Whickman, but leaving behind my
personal email address to answer any questions about the design or
operation of the XP family of drivers.

Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>