documentation/iommu: Add note on existing DT binding status

The current dt binding for Exynos System MMU can be changed, if found
incompatible with the support for "Generic IOMMU Binding".
This patch adds a note to the binding documentation stating the same.

Signed-off-by: Shaik Ameer Basha <shaik.ameer@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>