perf: Introduce positive capability for raw events

Only a handful of CPU PMUs accept PERF_TYPE_{RAW,HARDWARE,HW_CACHE}
events without registering themselves as PERF_TYPE_RAW in the first
place. Add an explicit opt-in for these special cases, so that we can
make life easier for every other driver (and probably also speed up the
slow-path search) by having perf_try_init_event() do the basic type
checking to cover the majority of cases.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
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A further possibility is to automatically add the cap to PERF_TYPE_RAW
PMUs in perf_pmu_register() to have a single point-of-use condition; I'm
undecided...