Add optional dma-coherent property to binding doc.

Found by 'make dtbs_check' on arm64/amlogic DT files.
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210421204833.18523-2-khilman@baylibre.com'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>