Update i2c_emul.c to support i2c_target_register and i2c_target_unregister
function calls as well as support address forwarding in emulation.
Address forwarding helps us test IPCs in native sim. Instead of having to
emulate 2 separate cores, we can forward read/write requests from one bus
to another bus (effectively creating a loop). This way the same image can
simulate both the controller and the target.
Signed-off-by: Yuval Peress <peress@google.com>
