feat(arm): update documentation for cot-dt2c

Add documentation for the cot-dt2c feature

Change-Id: I27383882b639e39217d09ca76e316098cc4753d0
Signed-off-by: Xialin Liu <Xialin.Liu@ARM.com>
diff --git a/docs/tools/index.rst b/docs/tools/index.rst
index 91651bf..c0e214a 100644
--- a/docs/tools/index.rst
+++ b/docs/tools/index.rst
@@ -7,6 +7,7 @@
 
    memory-layout-tool
    transfer-list-compiler
+   cot-dt2c
 
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