arm64: perf: Simplify the ARMv8 PMUv3 event attributes

For each PMU event, there is a ARMV8_EVENT_ATTR(xx, XX) and
&armv8_event_attr_xx.attr.attr. Let's redefine the ARMV8_EVENT_ATTR
to simplify the armv8_pmuv3_event_attrs.

Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
[will: Dropped unnecessary array syntax]
Signed-off-by: Will Deacon <will@kernel.org>