msm: iommu: Create a Kconfig item for the IOMMU driver

Break the IOMMU driver out as a Kconfig item. Initially it
was decided to always build this in for 8x60, but this
driver is not strictly necessary and should be optionally
selectable.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>