iommu: io-pgtable: use size_t for unmap return value

Unmap returns a size_t all throughout the IOMMU framework.  Make
io-pgtable match this convention.

Change-Id: Ice4c75a428f0f95a665e2fbe4210349d6f78c220
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>