ACPI: Add documentation for exposing MRRM data

Initial implementation provides enumeration of the address ranges
NUMA node numbers, and BIOS assigned region IDs for each range.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://patch.msgid.link/20250505173819.419271-4-tony.luck@intel.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: aegl

Documentation/ABI/testing/sysfs-firmware-acpi

@@ -248,3 +248,24 @@ Description:
 		  # cat ff_pwr_btn
 		  7	enabled
 
+What:		/sys/firmware/acpi/memory_ranges/rangeX
+Date:		February 2025
+Contact:	Tony Luck <tony.luck@intel.com>
+Description:
+		On systems with the ACPI MRRM table reports the parameters for
+		each range.
+
+		base: Starting system physical address.
+
+		length: Length of this range in bytes.
+
+		node: NUMA node that this range belongs to. Negative numbers
+		indicate that the node number could not be determined (e.g
+		for an address range that is reserved for future hot add of
+		memory).
+
+		local_region_id: ID associated with access by agents
+		local to this range of addresses.
+
+		remote_region_id: ID associated with access by agents
+		non-local to this range of addresses.