dt-bindings: dma: allwinner,sun50i-a64-dma: Add R40 compatible

R40 has DMA engine which is basically the same as that in A64, with only
known difference being number of request sources and number of channels.

Add compatible 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/20200825100030.1145356-2-jernej.skrabec@siol.net