lantiq: use external pci clock on ARV7506PW11

Fixes a ralink wireless driver related kernel crash as soon as a lot of
data is tranfered via wireless.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/lantiq/dts/ARV7506PW11.dts

@@ -83,6 +83,7 @@
 
 		pci@E105400 {
 			status = "okay";
+			lantiq,external-clock;
 			gpio-reset = <&gpio 21 GPIO_ACTIVE_HIGH>;
 		};
 	};