dt-bindings: drm/msm/gpu: Increase iommus maxItems

The apq8016 has two iommu handles not one. Increase the documented
maxItems to match.

Fixes: 6427f5d0 ("dt-bindings: drm/msm/gpu: convert to YAML")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>