defconfig: arm64: Enable DEBUG_PREEMPT config on kona target

Enable DEBUG_PREEMPT on kona target so that this feature provides
"Preemption disabled at:" print information which is helpful for debugging.

Change-Id: Ic5af3abc09ab6237331c3005958c7e46a3049966
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>