ANDROID: iommu/io-pgtable: Allow IOMMU drivers to allocate/free page-table memory

The page-table memory is currently allocated and freed by the io-pgtable
layer, and there is no intervention from the IOMMU driver during this
process. However, in some environments, where the IOMMU driver may need
to be aware of the pages that are used for its page-tables, it is useful
to allow the IOMMU driver to manage the allocation and freeing of the
page table memory.

Thus, add support in the io-pgtable layer to allow IOMMU drivers to
allocate and free page-table memory.

Bug: 147086566
Change-Id: I7b38a72da1654273a4312e04c776b10612c57bdc
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>