iommu/exynos: Support multiple attach_device calls

IOMMU core calls attach_device callback without detaching device from
the previous domain. This patch adds support for such unballanced calls.

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