Make sure that we have access to the performance counters and
that they aren't being used by perf events or anything else.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Jean Pihet <jpihet@mvista.com>
Signed-off-by: Jamie Iles <jamie.iles@picochip.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>