dt-bindings: ddr: Rename lpddr2 directory

Change directory name to be ready for new types of memories.
Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

Documentation/devicetree/bindings/lpddr2/lpddr2.txt → Documentation/devicetree/bindings/ddr/lpddr2.txt

@@ -36,7 +36,7 @@ Child nodes:
   "lpddr2-timings" provides AC timing parameters of the device for
   a given speed-bin. The user may provide the timings for as many
   speed-bins as is required. Please see Documentation/devicetree/
-  bindings/lpddr2/lpddr2-timings.txt for more information on "lpddr2-timings"
+  bindings/ddr/lpddr2-timings.txt for more information on "lpddr2-timings"
 
 Example: