ia64: Hide the archdata.iommu field behind generic IOMMU_API

There is a generic, kernel wide configuration symbol for enabling the
IOMMU specific bits: CONFIG_IOMMU_API.  Implementations (including
INTEL_IOMMU driver) select it so use it here as well.

This makes the conditional archdata.iommu field consistent with other
platforms and also fixes any compile test builds of other IOMMU drivers,
when INTEL_IOMMU is not selected).

For the case when INTEL_IOMMU and COMPILE_TEST are not selected, this
should create functionally equivalent code/choice.  With COMPILE_TEST
this field could appear if other IOMMU drivers are chosen but
INTEL_IOMMU not.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: e93a1695

 ("iommu: Enable compile testing for some of drivers")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

Link: https://lore.kernel.org/r/20200518120855.27822-1-krzk@kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>