PCM: Add string conversion helper functions for chmap

PCM: Add string conversion helper functions for chmap

Added a few helper functions between chmap and string.
  snd_pcm_chmap_type_name() -- a string of the given chmap type
  snd_pcm_chmap_name() -- a string of the given channel position
  snd_pcm_chmap_print() -- print channel map on the given buffer
  snd_pcm_chmap_from_string() -- get a channel position from string
  snd_pcm_parse_string() -- parse the whole channel map from string

Signed-off-by: Takashi Iwai <tiwai@suse.de>