* Spreadtrum sound audio pipe driver

This node models spreadtrum sound audio pipe driver

Required properties:

- compatible  : "sprd,audio_pipe_voice","sprd,audio_pipe_effect",
	"sprd,audio_pipe_recordproc","sprd,audio_pipe_bthal".
- sprd,writesync :  synchronous or asynchronous for pipe write,
	we can not dependent user space f_flags O_NONBLOCK to decide
	if write to pipe is synchronous or asynchronous, driver guarantee it.
	1 write operation is synchronous
	0 write operation is asynchronous
- sprd,maxuserwritebuffer: buffer max size for userwritebuffer,
	user data size is not fix, we would not alloc too many memory
	to adept all user data size.
- sprd,channel : communication channel with audio co-processor,
	the channel number is defined in "sprd,audio_pipe_voice"
	is for voice process, in "sprd,"audio_pipe_effect"
	is for audio effect function, in "sprd,audio_pipe_recordproc"
	is for audio record process function. These channel number is
	negotiated with audio co-processor.
Example:

audio_pipe_voice {
	compatible = "audio_pipe_voice";
	sprd,writesync = <0>;
	sprd,maxuserwritebufsize = <0>;
	sprd,channel = <2>;
};
