dtb: Fix zcu102 bare-metal

A holdover from the dtsi model was getting both mailbox configs in the
bare-metal dtb for zcu102.

Fix this by making the zcu102-openamp overlay only disable the serial
port and not include the zynqmp-openamp overlay defs.

Signed-off-by: Bill Mills <bill.mills@linaro.org>

examples/linux/dts/xilinx/zcu102-openamp.dtso

@@ -1,7 +1,7 @@
 /* Changes specific to zcu102 but common to mode */
 
-/* include everything from generic zynqmp-openamp overlay */
-#include "zynqmp-openamp.dtso"
+/dts-v1/;
+/plugin/;
 
 /* Plus make these specific zcu102 changes */
 &{/axi} {