dt-bindings: xen: Clarify "reg" purpose

Xen on Arm has gained new support recently to calculate and report
extended regions (unused address space) safe to use for external
mappings. These regions are reported via "reg" property under
"hypervisor" node in the guest device-tree. As region 0 is reserved
for grant table space (always present), the indexes for extended
regions are 1...N.

No device-tree bindings update is needed (except clarifying the text)
as guest infers the presence of extended regions from the number
of regions in "reg" property.

While at it, remove the following sentence:
"This property is unnecessary when booting Dom0 using ACPI."
for "reg" and "interrupts" properties as the initialization is not
done via device-tree "hypervisor" node in that case anyway.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1639080336-26573-7-git-send-email-olekstysh@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

Author: Oleksandr Tyshchenko

Documentation/devicetree/bindings/arm/xen.txt

@@ -7,15 +7,17 @@ the following properties:
 	compatible = "xen,xen-<version>", "xen,xen";
   where <version> is the version of the Xen ABI of the platform.
 
-- reg: specifies the base physical address and size of a region in
-  memory where the grant table should be mapped to, using an
-  HYPERVISOR_memory_op hypercall. The memory region is large enough to map
-  the whole grant table (it is larger or equal to gnttab_max_grant_frames()).
-  This property is unnecessary when booting Dom0 using ACPI.
+- reg: specifies the base physical address and size of the regions in memory
+  where the special resources should be mapped to, using an HYPERVISOR_memory_op
+  hypercall.
+  Region 0 is reserved for mapping grant table, it must be always present.
+  The memory region is large enough to map the whole grant table (it is larger
+  or equal to gnttab_max_grant_frames()).
+  Regions 1...N are extended regions (unused address space) for mapping foreign
+  GFNs and grants, they might be absent if there is nothing to expose.
 
 - interrupts: the interrupt used by Xen to inject event notifications.
   A GIC node is also required.
-  This property is unnecessary when booting Dom0 using ACPI.
 
 To support UEFI on Xen ARM virtual platforms, Xen populates the FDT "uefi" node
 under /hypervisor with following parameters: