Clarify comment on JIT cache flush issue

The original fix (#304947) is needed, but the comment was suggesting a
hardware cause and a link with A53 errata. In all likelihood neither
of these is true, and the issue is actually caused by a kernel issue
affecting many Qualcomm devices (like those using the msm tree).

Change-Id: Ie750e6fd9eb7130982d184c22917b5c3ea4beed0