hw/i3c: Add Mock target

Adds a simple i3c device to be used for testing in lieu of a real
device.

The mock target supports the following features:
- A buffer that users can read and write to.
- CCC

hw/i3c: Add Mock target

Adds a simple i3c device to be used for testing in lieu of a real
device.

The mock target supports the following features:
- A buffer that users can read and write to.
- CCC support for commonly used CCCs when probing devices on an I3C bus.
- IBI sending upon receiving a user-defined byte.

Signed-off-by: Joe Komlodi <komlodi@google.com>

Reviewed-by: Titus Rwantare <titusr@google.com>
Reviewed-by: Patrick Venture <venture@google.com>
Link: https://lore.kernel.org/qemu-devel/20250613000411.1516521-18-komlodi@google.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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