documentation: iommu: Add binding document of Exynos System MMU

This patch adds a description of the device tree binding for the
Samsung Exynos System MMU.

Signed-off-by: Cho KyongHo <pullip.cho@samsung.com>
Signed-off-by: Shaik Ameer Basha <shaik.ameer@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>