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>

Documentation/devicetree/bindings/arm/cpus.txt

@@ -192,6 +192,7 @@ nodes to be present and contain the properties described below.
 			    "brcm,brahma-b15"
 			    "marvell,armada-375-smp"
 			    "marvell,armada-380-smp"
+			    "marvell,armada-390-smp"
 			    "marvell,armada-xp-smp"
 			    "qcom,gcc-msm8660"
 			    "qcom,kpss-acc-v1"