Documentation: bindings: berlin: consider our dt bindings as unstable

Because the support of Marvell Berlin SoCs is still a work in progress,
add a statement to explicitly consider our device tree files and
bindings as unstable.
Signed-off-by: NAntoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: NSebastian Hesselbarth <sebastian.hesselbarth@gmail.com>

Documentation/devicetree/bindings/arm/marvell,berlin.txt

 Marvell Berlin SoC Family Device Tree Bindings
 ---------------------------------------------------------------
 
+Work in progress statement:
+
+Device tree files and bindings applying to Marvell Berlin SoCs and boards are
+considered "unstable". Any Marvell Berlin device tree binding may change at any
+time. Be sure to use a device tree binary and a kernel image generated from the
+same source tree.
+
+Please refer to Documentation/devicetree/bindings/ABI.txt for a definition of a
+stable binding/ABI.
+
+---------------------------------------------------------------
+
 Boards with a SoC of the Marvell Berlin family, e.g. Armada 1500
 shall have the following properties: