ARM: mvebu: add enable-method property for CPUs

This commit updates the Armada XP Device Trees (for the three variants
of Armada XP) to declare the "enable-method" property for the CPUs,
which helps operating systems find the appropriate logic to manage the
CPUs, especially to boot secondary CPUs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1397483648-26611-4-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>