iommu: fix initialization without 'add_device' callback

IOMMU drivers can be initialized from of_iommu helpers. Such drivers don't
need to provide device_add callbacks to operate properly, so there is no
need to fail initialization if the callback is missing.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>