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>
diff --git a/sound/pci/hda/patch_sigmatel.c b/sound/pci/hda/patch_sigmatel.c
index 7e531d5..5b7c173 100644
--- a/sound/pci/hda/patch_sigmatel.c
+++ b/sound/pci/hda/patch_sigmatel.c
@@ -4494,7 +4494,7 @@
 		return err;
 
 	spec = codec->spec;
-	codec->power_mgmt = 1;
+	codec->power_save_node = 1;
 	spec->linear_tone_beep = 0;
 	spec->gen.mixer_nid = 0x1d;
 	spec->have_spdif_mux = 1;
@@ -4600,7 +4600,7 @@
 	codec->epss = 0; /* longer delay needed for D3 */
 
 	spec = codec->spec;
-	codec->power_mgmt = 1;
+	codec->power_save_node = 1;
 	spec->linear_tone_beep = 0;
 	spec->gen.own_eapd_ctl = 1;
 	spec->gen.power_down_unused = 1;
@@ -4650,7 +4650,7 @@
 	codec->epss = 0; /* longer delay needed for D3 */
 
 	spec = codec->spec;
-	codec->power_mgmt = 1;
+	codec->power_save_node = 1;
 	spec->linear_tone_beep = 0;
 	spec->gen.own_eapd_ctl = 1;
 	spec->gen.power_down_unused = 1;
@@ -4692,7 +4692,7 @@
 		return err;
 
 	spec = codec->spec;
-	codec->power_mgmt = 1;
+	codec->power_save_node = 1;
 	spec->linear_tone_beep = 0;
 	spec->gen.own_eapd_ctl = 1;
 	spec->gen.power_down_unused = 1;