perf events, powerpc: Add POWER7 stalled-cycles-frontend/backend events

Extent the POWER7 PMU driver with definitions for generic front-end and back-end
stall events.

As explained in Ingo's original comment(8f622422
), the exact definitions of the stall events are very much processor specific as

different things mean different in their respective instruction pipeline. These
two Power7 raw events are the closest approximation to the concept detailed in
Ingo's comment.

[PERF_COUNT_HW_STALLED_CYCLES_FRONTEND] = 0x100f8, /* GCT_NOSLOT_CYC */
It means cycles when the Global Completion Table has no slots from this thread

[PERF_COUNT_HW_STALLED_CYCLES_BACKEND] = 0x4000a,  /* CMPLU_STALL */
It means no groups completed and GCT not empty for this thread
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>