MAINTAINERS: Cleanup SGI-related entries

UV platforms are the only ones which currently use the XP/XPC/XPNET
driver, so it seems fair HPE should take some responsibility as
maintainers of it; so add Mike Travis and Steve Wahl. Cliff Whickman's
email address is no longer valid, so remove it. Robin Holt was contacted
and wishes to remain as a maintainer.

Update Dimitri Sivanich's email address for the SGI GRU driver.

Add Mike Travis to HPE Superdome Flex (UV) platform.

 [ bp: Massage commit message. ]

Acked-by: Robin Holt <robinmholt@gmail.com>
Signed-off-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201019203533.GA1203217@swahl-home.5wahls.com