MAINTAINERS: update info for sparse

Update the info for sparse. More specifically:

 - change W entry to point to sparse.docs.kernel.org

 - add Q & B entry (patchwork & bugzilla)

Link: http://lkml.kernel.org/r/20200621144204.53938-1-luc.vanoostenryck@gmail.com
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>