iommu/exynos: Ensure that SYSMMU is added only once to its master device

This patch prepares Exynos IOMMU driver for deferred probing
support. Once it gets added, of_xlate() callback might be called
more than once for the same SYSMMU controller and master device
(for example it happens when masters device driver fails with
EPROBE_DEFER). This patch adds a check, which ensures that SYSMMU
controller is added to its master device (owner) only once.

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