iommu/io-pgtable-arm: Use right-shift for division

Since entry_size is always a power of 2, use appropriate
right-shift operation for division instead. Also, this change
helps to be compatible with 32-bit machines where lib support
for division of unsigned long long values are not supported.

Change-Id: I74d9656206126599b9e4052ee6c158f23da298de
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>