axfer: add support for a mapper for multiple target

axfer: add support for a mapper for multiple target

This commit adds support of mapper for 'multiple' target. This handles
several files via 'container' functions, and constructs data frame buffer
for playback, or splits data frames from data frame buffer for capture.
When playback source files includes data frames with several channels, the
first channel is used to construct buffer. For capture direction, each of
channel of data frame is stored in one file, thus the file includes one
channel of data frame. When handling non-interleaved buffer, a caller
should use an array of buffer for each of channels with non-interleaved
data frames.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>