perf: arm_spe: Expose event filter

Expose an "event_filter" entry in the caps folder to inform user space
about which events can be filtered.

Change the return type of arm_spe_pmu_cap_get() from u32 to u64 to
accommodate the added event filter entry.

Signed-off-by: Leo Yan <leo.yan@arm.com>
Tested-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>