dt-bindings: loongarch: Add Loongson SoC boards compatibles

Add Loongson SoC boards binding with DT schema format using json-schema.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>