Add a sysfs cpu_capacity attribute with which it is possible to read and
write (thus over-writing default values) CPUs capacity. This might be
useful in situations where values needs changing after boot.

The new attribute shows up as:

 /sys/devices/system/cpu/cpu*/cpu_capacity
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>