dt-bindings: dmaengine: Add Loongson LS2X APB DMA controller

Add Loongson LS2X APB DMA controller binding with DT schema
format using json-schema.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://lore.kernel.org/r/078307641077edaf46dd986c6d31cea15545a208.1702365725.git.zhoubinbin@loongson.cn


Signed-off-by: Vinod Koul <vkoul@kernel.org>