dt-bindings: iommu: Add documentation for rockchip iommu

Add binding documentation for Rockchip IOMMU.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Simon Xue <xxm@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>