MAINTAINERS: add maintainers for GRU, XPC, XPNET and XP

Add MAINTAINERS for GRU, XPNET, XPC and XP drivers.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>