perf_event, powerpc: Fix compilation after big perf_counter rename

This fixes two places in the powerpc perf_event (perf_counter) code
where 'list_entry' needs to be changed to 'group_entry', but were
missed in commit 65abc865

 ("perf_counter: Rename list_entry ->
group_entry, counter_list -> group_list").

This also changes 'event' back to 'counter' in a couple of
contexts:

* Field and function names that deal with the limited-function
  counters: it's really the hardware counters whose function is
  limited, not the events that they count.  Hence:

  MAX_LIMITED_HWEVENTS -> MAX_LIMITED_HWCOUNTERS
  limited_event -> limited_counter
  freeze/thaw_limited_events -> freeze/thaw_limited_counters

* The machine-specific PMU description struct (struct power_pmu): this
  renames 'n_event' back to 'n_counter' since it really describes how
  many hardware counters the machine has.  (Renaming this back avoids
  a compile error in each of the machine-specific PMU back-ends where
  they initialize their power_pmu struct.)

Signed-off-by: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@ozlabs.org
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <19128.4280.813369.589704@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>