iommu/arm-smmu: Remove [alloc/free]_pages_exact usage

The input argument to allocate the page table memory is the
order of the size of the allocation. The size given to
alloc_pages_exact and free_pages_exact is calculated from
the order, so the size is always a power of 2. Thus, there
is no benefit to using [alloc/free]_pages_exact, so replace
their usage with calls to [alloc/free]_pages_exact.

Change-Id: Ic299ad3a1e3885700d927c894786b83a82f48cff
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>