ARM: mvebu: register the cpuidle driver for the Armada XP SoCs

The cpuidle is a platform driver so we register the device just after
the initialization of the board in an arch_initcall.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1397488214-20685-12-git-send-email-gregory.clement@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>