defconfig: 8916: Enable forced 4KB mapping for IOMMU

For creating most real scenario to measure performance
of IOMMUs, enable to use only 4K mappings for the buffers.

Change-Id: I685615b9fd9d6977c8d379a2eaf0d6c235ee8635
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>