dt-bindings: sound: wm8994: document wlf,csnaddr-pd property

Add the wlf,csnaddr-pd property to allow the CS/Addr pull-down to be
controlled from the device tree.

Signed-off-by: Anthony Brandon <anthony@amarulasolutions.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>