Documentation: DT bindings: add doc for Altera's SoCFPGA platform

Document "altr,socfpga-cyclone5", "altr,socfpga-arria5", and
"altr,socfpga-arria10".
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/altera.txt

+Altera's SoCFPGA platform device tree bindings
+---------------------------------------------
+
+Boards with Cyclone 5 SoC:
+Required root node properties:
+compatible = "altr,socfpga-cyclone5", "altr,socfpga";
+
+Boards with Arria 5 SoC:
+Required root node properties:
+compatible = "altr,socfpga-arria5", "altr,socfpga";
+
+Boards with Arria 10 SoC:
+Required root node properties:
+compatible = "altr,socfpga-arria10", "altr,socfpga";