dt-bindings: dmaengine: Add Loongson Multi-Channel DMA controller

dt-bindings: dmaengine: Add Loongson Multi-Channel DMA controller

The Loongson-2K0300/Loongson-2K3000 have built-in multi-channel DMA
controllers, which are similar except for some of the register offsets
and number of channels.

Obviously, this is quite different from the APB DMA controller used in
the Loongson-2K0500/Loongson-2K1000, such as the latter being a
single-channel DMA controller.

To avoid cluttering a single dt-binding file, add a new yaml file.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/135802de72b84f643d0b0624f3f79f13777147a1.1772853681.git.zhoubinbin@loongson.cn
Signed-off-by: Vinod Koul <vkoul@kernel.org>