perf: fix arm64 build error

I'm seeing the following build error on arm64:

  In file included from util/event.c:3:0:
  util/event.h:95:17: error: 'PERF_REGS_MAX' undeclared here (not in a function)
    u64 cache_regs[PERF_REGS_MAX];
                 ^

This patch adds a PERF_REGS_MAX definition for arm64.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Jean Pihet <jean.pihet@linaro.org>