HOWTO: Updates on subsystem trees, patchwork, -next (vs. -mm)

One of the roles which -mm fulfilled some time ago (to offer an
integration testing ground) has been taken over by -next.  This is still
news to Documentation/HOWTO, so mention it there.

Also add a word on how patchwork is used to track patches as they make
their way into subsystem trees.  Remove some arbitrary links to
subsystem repositories; they can all be found in the MAINTAINERS
database.

Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>