ARM: perf: don't pretend to support counting of L1I writes

ARM has a harvard cache architecture and cannot write directly to the
I-side.

This patch removes the L1I write events from the cache map (which
previously returned *read* events in many cases).

Reported-by: Mike Williams <michael.williams@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>