cpufreq: CPPC: Add sysfs documentation for perf_limited

Add ABI documentation for the Performance Limited Register sysfs
interface in the cppc_cpufreq driver.

Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Lifeng Zheng <zhenglifeng1@huawei.com>
Link: https://patch.msgid.link/20260206142658.72583-8-sumitg@nvidia.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Sumit Gupta

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -327,6 +327,24 @@ Description:	Energy performance preference
 
 		This file is only present if the cppc-cpufreq driver is in use.
 
+What:		/sys/devices/system/cpu/cpuX/cpufreq/perf_limited
+Date:		February 2026
+Contact:	linux-pm@vger.kernel.org
+Description:	Performance Limited
+
+		Read to check if platform throttling (thermal/power/current
+		limits) caused delivered performance to fall below the
+		requested level. A non-zero value indicates throttling occurred.
+
+		Write the bitmask of bits to clear:
+
+		  - 0x1 = clear bit 0 (desired performance excursion)
+		  - 0x2 = clear bit 1 (minimum performance excursion)
+		  - 0x3 = clear both bits
+
+		The platform sets these bits; OSPM can only clear them.
+
+		This file is only present if the cppc-cpufreq driver is in use.
 
 What:		/sys/devices/system/cpu/cpu*/cache/index3/cache_disable_{0,1}
 Date:		August 2008