arm64: perf: Constify static attribute_group structs

The only usage of these is to put their addresses in an array of
pointers to const attribute_group structs. Make them const to allow the
compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>