dmaengine: ls2x-apb: New driver for the Loongson LS2X APB DMA controller

The Loongson LS2X APB DMA controller is available on Loongson-2K chips.

It is a single-channel, configurable DMA controller IP core based on the
AXI bus, whose main function is to integrate DMA functionality on a chip
dedicated to carrying data between memory and peripherals in APB bus
(e.g. nand).

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Yingkun Meng <mengyingkun@loongson.cn>
Link: https://lore.kernel.org/r/8df2a0199434fba3535831082966c2442ecf1cae.1702365725.git.zhoubinbin@loongson.cn
Signed-off-by: Vinod Koul <vkoul@kernel.org>