drivers: iommu: add interrupt selftest for arm smmu

Add an interrupt selftest to make sure all the context interrupts
are configured properly. Saves validation time. To enable, set
CONFIG_ARM_SMMU_SELFTEST and pass arm_smmu.selftest=1 as kernel
boot arg.

Change-Id: I061a10be32d5fa13a9c4897ab5c81672f537ac4f
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>