mvebu: disable cpu idle on WRT1900ACv1

A regression seriously affecting the Linksys WRT1900ACv1 (Mamba) was
introduced some time between the OpenWrt/LEDE v4.4 and v4.9 kernels.
The root cause has not yet been identified, despite many attempts for
more than a year. Disabling the SoC specific CPU idle support should
mitigate this issue.

The symptoms on an affected system are unwanted reboots at a variable
frequency. In many cases almost immediately after boot, causing a
bootloop.  This effectively disables support for Mamba on OpenWrt
with kernels > v4.4.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/mvebu/patches-4.14/230-armada-xp-linksys-mamba-broken-idle.patch

@@ -0,0 +1,10 @@
+--- a/arch/arm/boot/dts/armada-xp-linksys-mamba.dts
++++ b/arch/arm/boot/dts/armada-xp-linksys-mamba.dts
+@@ -563,3 +563,7 @@
+ 		};
+ 	};
+ };
++
++&coherencyfab {
++	broken-idle;
++};