defconfig: enable CONFIG_ARM_SMMU_SELFTEST for debug config

Enable CONFIG_ARM_SMMU_SELFTEST for debug config. The actual
interrupt self test runs only when arm_smmu.selftest is set to 1.

Change-Id: I040ba9d5d98ff2e9dbbc579e8a0f5062d9bedfd4
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>