doc: iot2050: Add a note about the watchdog firmware

This is enabled by default, thus should be described as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
diff --git a/doc/board/siemens/iot2050.rst b/doc/board/siemens/iot2050.rst
index cb49a0e..efe94a4 100644
--- a/doc/board/siemens/iot2050.rst
+++ b/doc/board/siemens/iot2050.rst
@@ -27,6 +27,10 @@
  - seboot_pg1.bin
  - seboot_pg2.bin
 
+When using the watchdog, a related firmware for the R5 core(s) is needed, e.g.
+https://github.com/siemens/k3-rti-wdt. The name and location of the image is
+configured via CONFIG_WDT_K3_RTI_FW_FILE.
+
 For building an image containing the OTP key provisioning data, below binary
 needs to be present in the build folder: