oprofile: disable on xtensa

Missing memory barrier definition. Fixes
http://autobuild.buildroot.net/results/df7/df797a81111fa15de9337adefca05cbac156d0b2/

.

Also, add nios2 to the dependencies comment while at it.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>