[SERIAL] Documentation/jsm.txt is a no show.

In kernel bugzilla #5176 (http://bugzilla.kernel.org/show_bug.cgi?id=5176)
Harry R\374ter <h.rueter@gmx.de> points out Documentation/jsm.txt is missing.

No one at Digi seems to care, so just remove the stale reference.

Signed-off-by: Arthur Othieno <apgo@patchbomb.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>