dt-bindings: sound: wm8994: document wlf,spkmode-pu property

Add the wlf,spkmode-pu property to control the Speaker Mode pull-up from
the device tree.

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

Documentation/devicetree/bindings/sound/wm8994.txt

@@ -57,6 +57,9 @@ Optional properties:
 
   - wlf,ldoena-always-driven : If present LDOENA is always driven.
 
+  - wlf,spkmode-pu : If present enable the internal pull-up resistor on
+    the SPKMODE pin.
+
 Example:
 
 wm8994: codec@1a {