dt-bindings: display: ti,j721e-dss: Add dt-schema yaml binding

Add dt-schema yaml bindig for J721E DSS, J721E version TI Keystone
Display SubSystem.

Version history:

v2: no change

v3: - reg-names: "wp" -> "wb"
    - Add ports node
    - Add includes to dts example
    - reindent dts example

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

v5: - itemize reg, clocks and interrupts properties' descriptions
    - there is no "vp" reg-name, only "wb" for write back

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

v7: no change

v8: no change

v9: 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/fcba52837808853a4b2bf729bd33bd986cbdadf4.1580129724.git.jsarha@ti.com