defconfig: 8974: Enable CONFIG_DEBUG_MUTEXES back

Mutex debugging was disabled earlier because of a deadlock issue with
spinlocks this debugging mechanism uses. We got a workaround in
TZ image now which configures WFE as NOP. So enabling this back.

Change-Id: If1ada00e5e5392d8a5c6c5192c4ba92838bb79f8
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>