devicetree: bindings: add new SMP enable method for Marvell Armada 39x

This commit updates the ARM CPUs Device Tree binding to document a new
enable method of Marvell Armada 39x processors.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>