documentation: add a section on devicetree refs

There is no existing documentation on how `device ref` and aliases work
in the devicetree, and the behavior around devices not being in the same
location is difficult to discern as well as somewhat unexpected.

This should help prevent confusion leading to bugs such as the one fixed
by https://review.coreboot.org/c/coreboot/+/57298

Change-Id: I4b30f7d531cfc3453d6523a76084f1969125b4bf
Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57354
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/acpi/devicetree.md

@@ -20,6 +20,62 @@ devicetree.  Note, not all mainboards will have the devicetree/overridetree
 distinction, and may only have a devicetree.cb file.  Or you can always just
 write the ASL (ACPI Source Language) code yourself.
 
+### Naming and referencing devices
+
+When declaring a device, it can optionally be given an alias that can be
+referred to elsewhere. This is particularly useful to declare a device in one
+device tree while allowing its configuration to be more easily changed in an
+overlay. For instance, the AMD Picasso SoC definition
+(`soc/amd/picasso/chipset.cb`) declares an IOMMU on a PCI bus that is disabled
+by default:
+
+```
+chip soc/amd/picasso
+	device domain 0 on
+		...
+		device pci 00.2 alias iommu off end
+		...
+	end
+end
+```
+
+A device based on this SoC can override the configuration for the IOMMU without
+duplicating addresses, as in
+`mainboard/google/zork/variants/baseboard/devicetree_trembyle.cb`:
+
+```
+chip soc/amd/picasso
+	device domain 0
+		...
+		device ref iommu on end
+		...
+	end
+end
+```
+
+In this example the override simply enables the IOMMU, but it could also
+set additional properties (or even add child devices) inside the IOMMU `device`
+block.
+
+---
+
+It is important to note that devices that use `device ref` syntax to override
+previous definitions of a device by alias must be placed at **exactly the same
+location in the device tree** as the original declaration. If not, this will
+actually create another device rather than overriding the properties of the
+existing one. For instance, if the above snippet from `devicetree_trembyle.cb`
+were written as follows:
+
+```
+chip soc/amd/picasso
+	# NOTE: not inside domain 0!
+	device ref iommu on end
+end
+```
+
+Then this would leave the SoC's IOMMU disabled, and instead create a new device
+with no properties as a direct child of the SoC.
+
 ## Device drivers
 
 Let's take a look at an example entry from