iommu/io-pgtable: Add memory stats debugfs file

It can be useful for debugging to know how much memory is being used for
IOMMU page tables.  Add some dedicated allocation functions and a
debugfs file for this.

Change-Id: Id69b4b1b5df5dcc6c604eec3a12a894b8eab0eb6
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>