samples: tracing: make example in readme work

Add mps2_an521.overlay so that the example in the README.txt will
actually build.  Update README.txt to document needing to have
'zephyr,tracing-uart' property set under chosen node in devicetree.

Fixes #48416

Signed-off-by: Kumar Gala <galak@kernel.org>

samples/subsys/tracing/README.txt

@@ -18,6 +18,9 @@ or:
 
     cmake -DBOARD=mps2_an521 -DCONF_FILE=prj_uart_ctf.conf ..
 
+NOTE: You may need to set 'zephyr,tracing-uart' property under the chosen
+node in your devicetree.  See boards/mps2_an521.overlay for an example.
+
 After the application has run for a while, check the trace output file.
 
 --------------------------------------------------------------------------------

samples/subsys/tracing/boards/mps2_an521.overlay

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+/ {
+       chosen {
+               zephyr,tracing-uart = &uart1;
+       };
+};