ARM: sunxi_defconfig: enable CONFIG_REGULATOR_FIXED_VOLTAGE

I missed in 9a2ad529 that REGULATOR_FIXED_VOLTAGE had also gotten
deselected, so it needs to be added back as an explicit option.
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/configs/sunxi_defconfig

@@ -76,6 +76,7 @@ CONFIG_WATCHDOG=y
 CONFIG_SUNXI_WATCHDOG=y
 CONFIG_MFD_AXP20X=y
 CONFIG_REGULATOR=y
+CONFIG_REGULATOR_FIXED_VOLTAGE=y
 CONFIG_REGULATOR_GPIO=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y