sunxi: add defconfig for nanopi_duo2

FriendlyElec NanoPi Duo2 is a tiny SBC with Allwinner H3 and
Ampak AP6212 WiFi module. The device-tree for it is already
available in u-boot source tree. Add a default config for it.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
[Andre: enable USB gadgets]
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>