ar71xx-generic: add support for GL.INet GL-AR300M (#1163)

targets/ar71xx-generic

@@ -60,6 +60,8 @@ device gl-inet-6416a-v1 gl-inet-6416A-v1
 
 device gl-ar150 gl-ar150
 factory
+device gl-ar300m gl-ar300m
+factory
 
 
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