iommu: Init iommu-groups support earlier, in core_initcall

iommu_group_alloc might be called very early in case of iommu controllers
activated from of_iommu, so ensure that this part of subsystem is ready
when devices are being populated from device-tree (core_initcall seems to
be okay for this case).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Joerg Roedel <jroedel@suse.de>