Documentation: omap-twl4030: Move ti,codec property to optional

ti,codec property is not used in omap-twl4030 driver in linux kernel but
we keep it as optional property, so that the existing dtbs do not
become noncompliant after the change on other OS.

Signed-off-by: Marek Belisko <marek@goldelico.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>