[media] Documentation: DT: add bindings for ST DELTA

This patch adds DT binding documentation for STMicroelectronics
DELTA V4L2 video decoder.

Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>