iommu/io-pgtable-arm: Don't leave dangling table entries

Currently, when all of the 4K PTEs beneath a 2M table entry are
unmapped, that 2M table entry is left intact, even though it doesn't
point to any valid 4K mappings anymore.  This results in a warning if a
subsequent block mapping lands on top of the dangling table entry, since
we require empty page table entries when we map.  It also causes the
page at which that the stomped-on table was pointing to be leaked.  Fix
this by keeping track of how many entries are currently mapped beneath a
table.  When the map count goes to zero (in unmap), free up the page the
table is pointing at and zero out the table entry.

Change-Id: I470e6ffb2206a09fe7c24253e3fd64a744337a7f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>