ALSA: hda - Rename power_mgmt flag with power_save_node

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: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>