iommu: Refactor IOMMU pagetable page accounting into a config option

Refactor IOMMU pagetable page accounting code into a config option
so that it only includes additional headers in configurations where
the page accounting feature is needed. Doing so prevents the
definition of certain structures from being expanded, which can
affect the CRC value generated for exported symbols in module.symvers.

Change-Id: Idd6fc0e1cbb46ace211971ac142be491d0a5ab09
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>