arm64: dts: Add DMA device node for Spreadtrum SC9860

The Spreadtrum SC9860 platform has two DMA controllers, one is located
on the ap-ahb system, and another one is located on the agcp system.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>