dt-bindings: display: ti,k2g-dss: Add dt-schema yaml binding

Add dt-schema yaml bindig for K2G DSS, an ultra-light version of TI
Keystone Display SubSystem.

Version history:

v2: no change

v3: - Add ports node
    - 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
    - Remove ports node

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

v9: - Remove ports-node from the dts example

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/270297321f0768c10e241d289e3ac10e39cf12a9.1580129724.git.jsarha@ti.com