Only the PowerPC e500-based platforms use the MPC I2C controller.
Do not build it for the other machines.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191231183216.6781-7-philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>