test: Add i2c emulation for targets

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>

boards/native/native_sim/native_sim.dts

@@ -114,6 +114,7 @@
 		clock-frequency = <I2C_BITRATE_STANDARD>;
 		#address-cells = <1>;
 		#size-cells = <0>;
+		#forward-cells = <1>;
 		reg = <0x100 4>;
 	};