Documentation: admin-guide: pm: Add documentation for die_id

Add documentation to describe die_id attribute.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250508230250.1186619-6-srinivas.pandruvada@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Author: spandruvada

Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst

@@ -91,6 +91,11 @@ Attributes in each directory:
 ``domain_id``
 	This attribute is used to get the power domain id of this instance.
 
+``die_id``
+	This attribute is used to get the Linux die id of this instance.
+	This attribute is only present for domains with core agents and
+	when the CPUID leaf 0x1f presents die ID.
+
 ``fabric_cluster_id``
 	This attribute is used to get the fabric cluster id of this instance.