defconfig: remove support for /dev/mem and /dev/kmem on perf builds

Keeping /dev/mem and /dev/kmem exposed to userspace allows userspace
to peek/poke to these areas and is a security risk. Remove support for
/dev/mem and /dev/kmem on perf builds.

Change-Id: I1408b026e0c6830364cbb84133751acb3ff08aeb
CRs-Fixed: 530719
Signed-off-by: Sarang Joshi <spjoshi@codeaurora.org>