iommu: io-pgtable-arm: Set child ref count in arm_lpae_split_blk_unmap

Currently in arm_lpae_split_blk_unmap when a table entry is created we do
not initialize the table's child ref count with the number of children.
This is a problem because the unmap code expects the table's child ref
count to be accurate.

Fix arm_lpae_split_blk_unmap to properly initialize the table's child ref
count.

Change-Id: Ief6ab5940968981967c9037c6a28087240973037
Signed-off-by: Liam Mark <lmark@codeaurora.org>