arm64: perf: don't expose CHAIN event in sysfs

The CHAIN event allows two 32-bit counters to be treated as a single
64-bit counter, under certain allocation restrictions on the PMU.

Whilst userspace could theoretically create CHAIN events using the raw
event syntax, we don't really want to advertise this in sysfs, since
it's useless in isolation. This patch removes the event from our /sys
entries.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>