ath79: add variant UniFi AP LR

The hardware difference is the antenna which has a higher gain compared
to the original UniFi AP.

The variant was supported before in ar71xx.

Signed-off-by: Jan-Niklas Burfeind <git@aiyionpri.me>

target/linux/ath79/dts/ar7241_ubnt_unifi-ap-lr.dts

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+
+#include "ar7241_ubnt_unifi-ap.dtsi"
+
+/ {
+	compatible = "ubnt,unifi", "qca,ar7241";
+	model = "Ubiquiti UniFi AP LR";
+};