iommu/exynos: Update device tree documentation

Exynos SYSMMU bindings documentation was merged before generic IOMMU
binding have been introduced. This patch updates documentation to match
current state.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>