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.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-kernel-iommu_groups

@@ -25,14 +25,10 @@ Description:    /sys/kernel/iommu_groups/reserved_regions list IOVA
 		the base IOVA, the second is the end IOVA and the third
 		field describes the type of the region.
 
-What:		/sys/kernel/iommu_groups/reserved_regions
-Date: 		June 2019
-KernelVersion:  v5.3
-Contact: 	Eric Auger <eric.auger@redhat.com>
-Description:    In case an RMRR is used only by graphics or USB devices
-		it is now exposed as "direct-relaxable" instead of "direct".
-		In device assignment use case, for instance, those RMRR
-		are considered to be relaxable and safe.
+		Since kernel 5.3, in case an RMRR is used only by graphics or
+		USB devices it is now exposed as "direct-relaxable" instead
+		of "direct". In device assignment use case, for instance,
+		those RMRR are considered to be relaxable and safe.
 
 What:		/sys/kernel/iommu_groups/<grp_id>/type
 Date:		November 2020