ARM: perf: add hook for event index clearing

On Krait processors we have a many-to-one relationship between
raw CPU events and the event programmed into the PMNx counter.
Two raw CPU events could map to the same value programmed in the
PMNx counter. To avoid this problem, we check for collisions
during the get_event_idx() callback by setting a bit in a bitmap
whenever a certain event is used in a PMNx counter (see the next
patch). Unfortunately, we don't have a hook to clear this bit in
the bitmap when the event is deleted so let's add an optional
clear_event_idx() callback for this purpose.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>