of: Add #address-cells/#size-cells in the device-tree root empty node

On systems where ACPI is enabled or when a device-tree is not passed to
the kernel by the bootloader, a device-tree root empty node is created.
This device-tree root empty node does not have the #address-cells and
the #size-cells properties

This leads to the use of the default address cells and size cells values
which are defined in the code to 1 for the address cells value and 1 for
the size cells value.

According to the devicetree specification and the OpenFirmware standard
(IEEE 1275-1994) the default value for #address-cells should be 2.

Also, according to the devicetree specification, the #address-cells and
the #size-cells are required properties in the root node.

The device tree compiler already uses 2 as default value for address
cells and 1 for size cells. The powerpc PROM code also uses 2 as default
value for address cells and 1 for size cells. Modern implementation
should have the #address-cells and the #size-cells properties set and
should not rely on default values.

On x86, this root empty node is used and the code default values are
used.

In preparation of the support for device-tree overlay on PCI devices
feature on x86 (i.e. the creation of the PCI root bus device-tree node),
the default value for #address-cells needs to be updated. Indeed, on
x86_64, addresses are on 64bits and the upper part of an address is
needed for correct address translations. On x86_32 having the default
value updated does not lead to issues while the upper part of a 64-bit
value is zero.

Changing the default value for all architectures may break device-tree
compatibility. Indeed, existing dts file without the #address-cells
property set in the root node will not be compatible with this
modification.

Instead of updating default values, add both required #address-cells
and #size-cells properties in the device-tree empty node.

Use 2 for both properties value in order to fully support 64-bit
addresses and sizes on systems using this empty root node.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20241202131522.142268-6-herve.codina@bootlin.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

drivers/of/empty_root.dts

@@ -2,5 +2,12 @@
 /dts-v1/;
 
 / {
-
+	/*
+	 * #address-cells/#size-cells are required properties at root node.
+	 * Use 2 cells for both address cells and size cells in order to fully
+	 * support 64-bit addresses and sizes on systems using this empty root
+	 * node.
+	 */
+	#address-cells = <0x02>;
+	#size-cells = <0x02>;
 };