David suggested that the name "power_mgmt" is too ambiguous.  Rename
the flag with a bit clearer one "power_save_node".

Also, add the corresponding description to HD-Audio.txt, too.
Reported-by: NDavid Henningsson <david.henningsson@canonical.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>