sunxi: Consider SPL size limitations for FIT loading

Now that 32-bit SoCs can load U-Boot proper (and possibly other firmware)
from a FIT, people can use that by enabling CONFIG_SPL_LOAD_FIT.
However SPL_FIT_IMAGE_TINY is required to stay within the 24 or 32 KiB
SPL size limit on early SoCs; for consistency, enable it everywhere.

Signed-off-by: Samuel Holland <samuel@sholland.org>
[Andre: drop unconditional FIT image enablement for all SoCs]
Reviewed-by: Andre Przywara <andre.przywara@arm.com>