Introduce an AspeedI2CBus SysBusDevice model and attach the associated
memory region and IRQ to the newly instantiated objects.
Before this change, the I2C bus IRQs were all attached to the
SysBusDevice model of the I2C controller. Adapt the AST2600 SoC
realize routine to take into account this change.
Signed-off-by: Cédric Le Goater <clg@kaod.org>