ARM: dts: sun8i: r40: Add DMA node

Allwinner R40 SoC has DMA with 16 channels and 31 request sources.

Add a node for it.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200825100030.1145356-3-jernej.skrabec@siol.net