sunxi: enable automatic FIT build for 64-bit SoCs

The Allwinner SoCs with 64-bit cores use an ARM Trusted Firmware binary,
which needs to be loaded alongside U-Boot proper.
Set the respective Kconfig options to let them select this feature and
also automatically build the FIT image.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[Rename Kconfig path to arch/arm/mach-sunxi/Kconfig]
Signed-off-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>