tests/avocado/machine_aspeed.py: Update SDK images

Switch to the latest v8.06 release which introduces interesting
changes for the AST2600 I2C and I3C models. Also take the AST2600 A2
images instead of the default since QEMU tries to model The AST2600 A3
SoC.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>