sunxi: axp221: Disable dcdc4 on sun8i (A23)

dcdc4 is not used on sun8i, disable it.

While at it also add comments to the other fixed voltages to document what
they are used for.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>

board/sunxi/board.c

@@ -176,10 +176,14 @@ void sunxi_board_init(void)
 #ifdef CONFIG_AXP221_POWER
 	power_failed = axp221_init();
 	power_failed |= axp221_set_dcdc1(CONFIG_AXP221_DCDC1_VOLT);
-	power_failed |= axp221_set_dcdc2(1200);
-	power_failed |= axp221_set_dcdc3(1200);
-	power_failed |= axp221_set_dcdc4(1200);
-	power_failed |= axp221_set_dcdc5(1500);
+	power_failed |= axp221_set_dcdc2(1200); /* A31:VDD-GPU, A23:VDD-SYS */
+	power_failed |= axp221_set_dcdc3(1200); /* VDD-CPU */
+#ifdef CONFIG_MACH_SUN6I
+	power_failed |= axp221_set_dcdc4(1200); /* A31:VDD-SYS */
+#else
+	power_failed |= axp221_set_dcdc4(0);    /* A23:unused */
+#endif
+	power_failed |= axp221_set_dcdc5(1500); /* VCC-DRAM */
 	power_failed |= axp221_set_dldo1(CONFIG_AXP221_DLDO1_VOLT);
 	power_failed |= axp221_set_dldo4(CONFIG_AXP221_DLDO4_VOLT);
 	power_failed |= axp221_set_aldo1(CONFIG_AXP221_ALDO1_VOLT);