nb,soc/amd/*/iommu: fix comment about IOMMU MMIO resource

This comment was added with the AMD family 15h Trinity IOMMU support in
commit 88ebbeb7e2 and looks like a copy of
the comment about the subtractive decode ranges in the LPC device. The
IOMMU doesn't have any subtractively decoded I/O or MMIO ranges and this
is also not what the code does. This resource is the MMIO region to
configure the IOMMU instead, so fix the comment in all copies of the
IOMMU support code.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I2e1e3a46b839b9e58b836932c1bc9b41b1b1dc02
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61631
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/northbridge/amd/agesa/family15tn/iommu.c

@@ -13,7 +13,7 @@ static void iommu_read_resources(struct device *dev)
 	/* Get the normal pci resources of this device */
 	pci_dev_read_resources(dev);
 
-	/* Add an extra subtractive resource for both memory and I/O. */
+	/* IOMMU MMIO registers */
 	res = new_resource(dev, 0x44);
 	res->size = 512 * 1024;
 	res->align = log2(res->size);

src/northbridge/amd/pi/00730F01/iommu.c

@@ -12,7 +12,7 @@ static void iommu_read_resources(struct device *dev)
 	/* Get the normal pci resources of this device */
 	pci_dev_read_resources(dev);
 
-	/* Add an extra subtractive resource for both memory and I/O. */
+	/* IOMMU MMIO registers */
 	res = new_resource(dev, 0x44);
 	res->size = 512 * 1024;
 	res->align = log2(res->size);

src/soc/amd/common/block/iommu/iommu.c

@@ -12,7 +12,7 @@ static void iommu_read_resources(struct device *dev)
 	/* Get the normal pci resources of this device */
 	pci_dev_read_resources(dev);
 
-	/* Add an extra subtractive resource for both memory and I/O. */
+	/* IOMMU MMIO registers */
 	res = new_resource(dev, 0x44);
 	res->size = 512 * KiB;
 	res->align = log2(res->size);