sunxi: defconfig: add supported DT list for Pine64

When a board uses a FIT image to load U-Boot proper, it requires a list
of supported device trees to be supplied in CONFIG_OF_LIST, from which it
chooses the right one at runtime.
For boards with just one possible DT (like the OrangePi PC2) this
defaults to CONFIG_DEFAULT_DEVICE_TREE, but for the Pine64 with its two
possible models we provide all compatible DTs in this config symbol.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@openedev.com>