It's all very well providing an events directory to userspace that
details our events in terms of "event=0xNN", but if we don't define how
to encode the "event" field in the perf attr.config, then it's a waste
of time.

This patch adds a single format entry to describe that the event field
occupies the bottom 10 bits of our config field on ARMv8 (PMUv3).
Signed-off-by: Will Deacon <will.deacon@arm.com>