mvebu: LS421DE: fix low performance issue

The Buffalo Linkstation LS421DE isn't able to enable the Level 2 cache
(AKA Aurora cache). As of result of this, the throughput is about half of
the expected, e.g when doing network data transfers.

Fix it by adding the broken-idle property in the coherency fabric node.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>

target/linux/mvebu/files/arch/arm/boot/dts/armada-370-buffalo-ls421de.dts

@@ -209,6 +209,10 @@
 	};
 };
 
+&coherencyfab {
+	broken-idle;
+};
+
 &eth1 {
 	pinctrl-0 = <&ge1_rgmii_pins>;
 	pinctrl-names = "default";