media: dt-bindings: allwinner: h6-vpu-g2: Add IOMMU reference property

The Hantro G2 video decoder block sits behind an IOMMU. The device tree
binding needs a property to reference it. Without a reference for the
implementation to properly configure the IOMMU, it will fault and cause
the video decoder to fail.

Add an "iommus" property for referring to the IOMMU port. The master ID
in the example is taken from the IOMMU fault error message on Linux,
and the number seems to match the order in the user manual's IOMMU
diagram.

Fixes: fd6be127 ("media: dt-bindings: allwinner: document H6 Hantro G2 binding")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>