ASoC: davinci-mcasp: Remove last reference to num-serializer in DT doc

Remove last reference to num-serializer in davinci-mcasp devicetree
binding document.
Signed-off-by: NJyri Sarha <jsarha@ti.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/davinci-mcasp-audio.txt

@@ -15,7 +15,6 @@ Required properties:
   	    IEC60958-1, and AES-3 formats.
 - tdm-slots : Slots for TDM operation. Indicates number of channels transmitted
   	      or received over one serializer.
-- num-serializer : Serializers used by McASP.
 - serial-dir : A list of serializer configuration. Each entry is a number
                indication for serializer pin direction.
                (0 - INACTIVE, 1 - TX, 2 - RX)