ARM: mvebu: add core support for Armada 39x

This commit adds the core support for Armada 39x, which is quite
simple:

 - a new Kconfig option which selects the appropriate clock and
   pinctrl drivers as well as other common features (GIC, L2 cache,
   SMP, etc.)

 - a new DT_MACHINE_START which references the top-level compatible
   strings supported for the Marvell Armada 39x.

 - a new SMP enable-method. The mechanism to enable CPUs for Armada
   39x appears to be the same as Armada 38x. However, we do not want
   to use marvell,armada-380-smp in the Device Tree, in the case of
   the discovery of a subtle difference in the future, which would
   require changing the Device Tree. And the enable-method isn't a
   compatible string: you can't specify several values and expect a
   fallback on the second string if the first one isn't
   supported. Therefore, we simply declare the SMP enable method
   "marvell,armada-390-smp" as doing the same thing as the
   "marvell,armada-380-smp" one.

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