powerpc/perf: Add stalled-cycles events

Support for stalled-cycles-frontend and stalled-cycles-backend is
added for e500-based processors.

The following mappings are used:

stalled-cycles-frontend or idle-cycles-frontend:
Com:18 Cycles decode stalled

stalled-cycles-backend or idle-cycles-backend
Com:19 cycles issue stalled

Signed-off-by: Chris Freehill <chrisf@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>