USB,USB USX2Y
Implements 'rawusb' pcm data transfer through hwdep interface:
The usb_hc moves pcm data from/into memory via DMA.
That memory is mmaped by jack's usx2y driver.
Jack's usx2y driver is the first/last executable code to read/write pcm data.
Read/write is a combination of power of 2 period shaping and float/int
conversation.

Compared to standard alsa/jack we leave out power of 2 period shaping
inside snd-usb-usx2y which needs memcpy() and additional buffers.
As a side effect possible unwanted pcm-data coruption resulting of
standard alsa's snd-usb-usx2y period shaping scheme falls away.
Result is sane jack operation at buffering schemes down to 128frames,
2 periods.

Also changed Kconfig file, so snd-usb-usx2y is only available for X86,
PPC or ALPHA platforms, as on others DMA-memory isn't mmapable.
Signed-off-by: Karsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>