ASoC: binding: add tdm-slot.txt

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/tdm-slot.txt

+TDM slot:
+
+This specifies audio DAI's TDM slot.
+
+TDM slot properties:
+dai-tdm-slot-num : Number of slots in use.
+dai-tdm-slot-width :  Width in bits for each slot.
+
+For instance:
+	dai-tdm-slot-num = <2>;
+	dai-tdm-slot-width = <8>;
+
+And for each spcified driver, there could be one .of_xlate_tdm_slot_mask()
+to specify a explicit mapping of the channels and the slots. If it's absent
+the default snd_soc_of_xlate_tdm_slot_mask() will be used to generating the
+tx and rx masks.
+
+For snd_soc_of_xlate_tdm_slot_mask(), the tx and rx masks will use a 1 bit
+for an active slot as default, and the default active bits are at the LSB of
+the masks.