arm: perf: Add event descriptions

Add additional information about the ARM architected hardware events
to make counters self describing. This makes the hardware PMUs easier
to use as perf list contains possible events instead of users having
to refer to documentation like the ARM TRMs.

Signed-off-by: Drew Richardson <drew.richardson@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>