arm: sunxi: Add AXP20X_ADC

AXP20X_POWER depends on IIO. Even though it does not depend on AXP20X_ADC,
it is the new, preferred way of getting power supply configuration, it's
going to be enabled anyway.
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/configs/sunxi_defconfig

@@ -129,6 +129,8 @@ CONFIG_DMADEVICES=y
 CONFIG_DMA_SUN6I=y
 # CONFIG_IOMMU_SUPPORT is not set
 CONFIG_EXTCON=y
+CONFIG_IIO=y
+CONFIG_AXP20X_ADC=y
 CONFIG_PWM=y
 CONFIG_PWM_SUN4I=y
 CONFIG_PHY_SUN4I_USB=y