mvebu: disable cpu idle on WRT1900ACv1

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>