DT: add documentation for the Ralink MIPS SoCs

Ralink MIPS SoC device tree bindings

1. SoCs

Each device tree must specify a compatible value for the Ralink SoC

it uses in the compatible property of the root node. The compatible

value must be one of the following values:

  ralink,rt2880-soc

  ralink,rt3050-soc

  ralink,rt3052-soc

  ralink,rt3350-soc

  ralink,rt3352-soc

  ralink,rt3883-soc

  ralink,rt5350-soc

  ralink,mt7620a-soc

  ralink,mt7620n-soc