allwinner: power: Add DCDC6 power rail

The DCDC6 power rail is typically driving VDD_SYS in the SoC, so it is
on by default and uses the default voltage.

As there seems to be at least on board using a different voltage, add
the rail to the list of known voltage lines, so we can setup the right
voltage as early as possible.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/allwinner/sun50i_a64/sunxi_power.c

@@ -183,6 +183,7 @@ struct axp_regulator {
 } regulators[] = {
 	{"dcdc1", 1600, 3400, 100, NO_SPLIT, 0x20, 0x10, 0},
 	{"dcdc5",  800, 1840,  10,       32, 0x24, 0x10, 4},
+	{"dcdc6",  600, 1520,  10,       50, 0x25, 0x10, 5},
 	{"dldo1",  700, 3300, 100, NO_SPLIT, 0x15, 0x12, 3},
 	{"dldo2",  700, 4200, 100,       27, 0x16, 0x12, 4},
 	{"dldo3",  700, 3300, 100, NO_SPLIT, 0x17, 0x12, 5},