CI: Add Aspeed AST2600

The AST2600 has a Qemu model that allows testing. Create a SPI NOR image
containing the combined SPL and u-boot FIT image.

Reviewed-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>