overlays: Fix miniuart-bt "krnbt" parameter

Although superficially appealing, an overlay parameter that targets a
label in the base DTB is not currently supported. Instead it is
necessary to create a fragment targeting the label which is patched
by the parameter.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

arch/arm/boot/dts/overlays/miniuart-bt-overlay.dts

@@ -81,7 +81,13 @@
 		};
 	};
 
+	fragment@7 {
+		target = <&minibt>;
+		minibt_frag: __overlay__ {
+		};
+	};
+
 	__overrides__ {
-		krnbt = <&minibt>,"status";
+		krnbt = <&minibt_frag>,"status";
 	};
 };