ARM: mvebu: add broken-idle option

The broken-idle option can be activated from the coherency-fabric DT
node. This property allows to disable the idle capability, when the
hardware doesn't support it, like the Seagate Personal Cloud boards.

Signed-off-by: Vincent Donnefort <vdonnefort@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>