media: dt-bindings: media: Document bindings for venus firmware device

Add devicetree binding documentation for firmware loader for video
hardware running on qualcomm chip.

Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Tested-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>