iommu/io-pgtable: fix __arm_lpae_free_pgtable leak

When unmapping 2MB mappings, which are 2MB aligned, the smmu driver
is leaking the 3rd level page tables.
Fix this leak by updating __arm_lpae_free_pgtable so that it no
longer leaks leaf table entries.

To reproduce this leak simply map and unmap a non-block 2MB mapping
which is 2MB aligned.

Change-Id: Ibdbdb084ceb8d03ebe0a04e8777e3eb9419e9b87
Signed-off-by: Liam Mark <lmark@codeaurora.org>