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

Altera's SoCFPGA platform device tree bindings

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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";