ASoC: binding: add 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.