iommu/iova: Allow compiling the library without IOMMU support

Drivers such as the Intel IPU3 ImgU driver use the IOVA library to manage
the device's own virtual address space while not implementing the IOMMU
API. Currently the IOVA library is only compiled if the IOMMU support is
enabled, resulting into a failure during linking due to missing symbols.

Fix this by defining IOVA library Kconfig bits independently of IOMMU
support configuration, and descending to the iommu directory
unconditionally during the build.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>