arm64: dts: ti: k3-am65: mark dss as dma-coherent

DSS is IO coherent on AM65, so we should mark it as such with
'dma-coherent' property in the DT file.

Fixes: fc539b90 ("arm64: dts: ti: am654: Add DSS node")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Nikhil Devshatwar <nikhil.nd@ti.com>
Cc: stable@vger.kernel.org # v5.8+
Link: https://lore.kernel.org/r/20201102134650.55321-1-tomi.valkeinen@ti.com