msm: iommu: Kconfig item for cacheable page tables

Add a Kconfig item to allow the IOMMU page tables to be
coherent in the L2 cache. This generally reduces IOTLB miss
latencies and has been shown to improve multimedia
performance.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>