kbuild: silence documentation GEN xml messages according to $(quiet)

Add rules for gen_xml and its quiet & silent variants.  This causes "make -s"
to be silent for gen_xml.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>