iommu/amd: Split amd_iommu_init function

The hardware-initializtion part of the AMD IOMMU driver is
split out into a seperate function. This function can now be
called either from amd_iommu_init() itself or any other
place if the hardware needs to be ready earlier. This will
be used to implement interrupt remapping for AMD.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>