dt-bindings: crypto: Add optional dma properties

Some platforms, like the v3s, have DMA channels assigned to the crypto
engine, which were in the DTSI but were never documented.

Let's make sure they are.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20211116143255.385480-1-maxime@cerno.tech