ath79: rename an engenius DTSI to generic senao name

Other vendors can use this DTSI, for example, WatchGuard
there are likely several brands that use the same board design
because of outsourcing hardware from Senao.

For example, Watchguard AP300
has the same hardware as Engenius EAP600
so we use ar9344_engenius_exx600.dtsi for that

Signed-off-by: Michael Pratt <mcpratt@pm.me>

target/linux/ath79/dts/ar9344_engenius_eap600.dts

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-or-later OR MIT
 
-#include "ar9344_engenius_exx600.dtsi"
+#include "ar9344_senao_ap-dual.dtsi"
 
 / {
 	model = "EnGenius EAP600";

target/linux/ath79/dts/ar9344_engenius_ecb600.dts

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-or-later OR MIT
 
-#include "ar9344_engenius_exx600.dtsi"
+#include "ar9344_senao_ap-dual.dtsi"
 
 / {
 	model = "EnGenius ECB600";