dt-bindings: display: sun8i-mixer: Allow for an iommu property

The H6 mixer is attached to an IOMMU, so let's allow that property to be
set in the bindings.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/7941e0c02794e6336da75fcac950ecd43be7fd97.1589378833.git-series.maxime@cerno.tech