dt-bindings: soc: nuvoton: Add NPCM GFXI

Add dt-bindings document for Graphics Core Information (GFXI) node. It
is used by NPCM video driver to retrieve Graphics core information.

Signed-off-by: Marvin Lin <milkfafa@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>