media: dt-bindings: qcom,sc7280-venus: Allow one IOMMU entry

Some SC7280-based boards crash when providing the "secure_non_pixel"
context bank, so allow only one iommu in the bindings also.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>