Revert "configs: disable build of iommu tools"

This reverts commit 052e11c6.

Linux kernel patches for SMMU Test Engine are now available and can be
used with the related smmute tools scripts to test I/O virtualization
use-case. So revert the change that disabled LINUX_TOOLS_IOMMU_BUILD
build config.

Change-Id: I506dee124c8bbd52c80fd33f4b6dec025d695c92
Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>