dt-bindings: MIPS: Require SoC compatible string after board string

All devicetree board files list a compatible string for the SoC after
the compatible string of the board.

Enhance the YAML format so that these SoC compatible strings appear
aside each supported Ingenic board.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Rob Herring <robh@kernel.org>