devices: add sm8550-hdk in the index

That allows for links from the sidebar to be created.

Change-Id: I657d12ed1cd4ce98eda1303ec9615fd946787931
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
diff --git a/docs/devices/index.rst b/docs/devices/index.rst
index 429e485..0e22baf 100644
--- a/docs/devices/index.rst
+++ b/docs/devices/index.rst
@@ -31,3 +31,9 @@
 
 	vim3
 
+.. toctree::
+        :maxdepth: 1
+        :caption: SM8550-HDK (WIP)
+
+	sm8550-hdk
+