dt-bindings: display: ti,am65x-dss: add missing properties to dt-schema

Add assigned-clocks, assigned-clock-parents and dma-coherent optional
properties.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200916131009.221252-2-tomi.valkeinen@ti.com