dt-bindings: MIPS: Document Ingenic SoCs binding.

Document the available properties for the SoC root node and the
CPU nodes of the devicetree for the Ingenic XBurst SoCs.

Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Tested-by: Paul Boddie <paul@boddie.org.uk>
Signed-off-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>