dt-bindings: display: ti,am65x-dss: Add dt-schema yaml binding

Add dt-schema yaml bindig for AM65x DSS, AM65x version TI Keystone
Display SubSystem.

Version history:

v2: no change

v3: - Add ports node
    - use allOf in ti,am65x-oldi-io-ctrl to add both $ref and maxItems
    - Add includes to dts example
    - reindent dts example

v4: - Add descriptions to reg and clocks properties
    - Remove minItems when its value is the same as maxItems value

v5: - itemize reg and clocks properties' descriptions

v6: - Add Reviewed-by: from Rob Herring <robh@kernel.org> and
      Benoit Parrot <bparrot@ti.com>

v7: no change

v8: no change

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1799656fb8cc42ca698fc29bbc80c1bae442d6d3.1580129724.git.jsarha@ti.com