ARM: shmobile: add sample amixer settings for ap4evb

FSI2A-AK4643 needs amixer setting to use it.
This patch adds quick setting guide
Signed-off-by: NKuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>

arch/arm/mach-shmobile/board-ap4evb.c

@@ -143,6 +143,10 @@
  *
  * SW41	:  ON : SH-Mobile AP4 Audio Mode
  *	: OFF : Bluetooth Audio Mode
+ *
+ * it needs amixer settings for playing
+ *
+ * amixer set "Headphone Enable" on
  */
 
 /*