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: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>