docs: readd TP-Link TD-W8970 v1

This device is already supported.
It's a lantiq device that was entered as ar71xx
in to the list of supported device and therefore
removed before the release of Gluon 22.

(cherry picked from commit 95e5d382)

docs/user/supported_devices.rst

@@ -237,6 +237,9 @@ lantiq-xrx200
   - FRITZ!Box 7362 SL [#eva_ramboot]_ [#lan_as_wan]_
   - FRITZ!Box 7412 [#eva_ramboot]_
 
+* TP-Link
+  - TD-W8970 (v1) [#lan_as_wan]_
+
 lantiq-xway
 -----------