This patch adds a "cpumask" attribute to CCN's event_source class sysfs
directory. Perf user tool uses it to restrict events to the
processor(s) enumerated in this mask.

This patch provides a single CPU mask, making it possible to run "-a"
perf session (previously it would request the same CCN event, for
example cycle counter, on each available core and most likely fail).
Initially the mask is set to the CPU that happened to probe the driver,
but it will be changed when it is hot-un-plugged (active events are
migrated to another CPU then).

Example:

 Performance counter stats for 'system wide':

CPU0               2968148      cycles
CPU1               2236736      cycles
CPU2               1797968      cycles
CPU3               1831715      cycles
CPU1            1201850868      ccn/cycles/

       1.001241383 seconds time elapsed
Signed-off-by: Pawel Moll <pawel.moll@arm.com>