iommu/io-pgtable-fast: optimize statically allocated pages

Presently fastmap iommu feature allocates page tables for
full 4GB virtual address space. This can be optimized to
consider virtual address range [base, size] needed by
client and prepare page tables only for applicable
region.

Change-Id: Ie6c23cb8e1702a823567e126f452b1e72d851f71
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>