iommu/vt-d: Switch from ioremap_cache to memremap

In preparation for deprecating ioremap_cache() convert its usage in
intel-iommu to memremap.  This also eliminates the mishandling of the
__iomem annotation in the implementation.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>