docs: ABI: iommu: remove duplicated definition for sysfs-kernel-iommu_groups

./scripts/get_abi.pl is reporting a duplicated definition for
/sys/kernel/iommu_groups/reserved_regions, both at the same
file:

	Warning: /sys/kernel/iommu_groups/reserved_regions is defined 2 times:  Documentation/ABI/testing/sysfs-kernel-iommu_groups:15  Documentation/ABI/testing/sysfs-kernel-iommu_groups:27

Fix it by merging those into an unified entry.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/ec33e8e9b8f120232ffb3b9fcc99c97b87f242e3.1621413933.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>