Documentation/ABI: add information about cpu_capacity

/sys/devices/system/cpu/cpu#/cpu_capacity describe information about
CPUs heterogeneity (ref. to Documentation/devicetree/bindings/arm/
cpu-capacity.txt).

Add such description.
Signed-off-by: NJuri Lelli <juri.lelli@arm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -366,3 +366,10 @@ Contact:	Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
 Description:	AArch64 CPU registers
 		'identification' directory exposes the CPU ID registers for
 		 identifying model and revision of the CPU.
+
+What:		/sys/devices/system/cpu/cpu#/cpu_capacity
+Date:		December 2016
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:	information about CPUs heterogeneity.
+
+		cpu_capacity: capacity of cpu#.