sunxi: enable missing config symbol

Enable CONFIG_MFD_AXP20X_I2C after the switch to Kernel 4.9, this symbol
is reportedly required to get USB working on the Banana Pi.

Ref: https://forum.lede-project.org/t/sunxi-kernel-4-9-usb-not-working/10513
Signed-off-by: Jo-Philipp Wich <[email protected]>

target/linux/sunxi/config-4.9

@@ -331,6 +331,7 @@ CONFIG_MDIO_SUN4I=y
 CONFIG_MEDIA_SUPPORT=y
 # CONFIG_MFD_AC100 is not set
 CONFIG_MFD_AXP20X=y
+CONFIG_MFD_AXP20X_I2C=y
 CONFIG_MFD_AXP20X_RSB=y
 CONFIG_MFD_CORE=y
 CONFIG_MFD_SUN6I_PRCM=y