dt-bindings: display: samsung,exynos7-decon: document iommus, memory-region, and ports

Similar to FIMD and Exynos5433's DECON, the Exynos7 DECON hardware:
- May optionally require an IOMMU to initialize a display region.
- Outputs image data to another block, say an MIC or a DSI master.

If an IOMMU is present, it may also require to access the reserved
framebuffer region.

Document these bindings in the devicetree schema.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>