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

Documentation/devicetree/bindings/display/ti/ti,am65x-dss.yaml

@@ -55,6 +55,14 @@ properties:
       - const: vp1
       - const: vp2
 
+  assigned-clocks:
+    minItems: 1
+    maxItems: 3
+
+  assigned-clock-parents:
+    minItems: 1
+    maxItems: 3
+
   interrupts:
     maxItems: 1
 
@@ -62,6 +70,9 @@ properties:
     maxItems: 1
     description: phandle to the associated power domain
 
+  dma-coherent:
+    type: boolean
+
   ports:
     type: object
     description: