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 <[e-mail]>
Acked-by: Charles Keepax <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/sound/wm8994.txt

···
   - wlf,spkmode-pu : If present enable the internal pull-up resistor on
     the SPKMODE pin.
 
+  - wlf,csnaddr-pd : If present enable the internal pull-down resistor on
+    the CS/ADDR pin.
+
 Example:
 
 wm8994: codec@1a {