configs: ast2600-evb: Enable SPL FIT support

Enable SPL FIT image load and verification support.
The HW accelerated SHA is also available with the
newly added support of the HACE HW hash engine.

The SPL thumb build is also enabled to keep the binary
less than 64KB to fit into the Aspeed secure boot design.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>