iommu/arm-smmu: Fix size parameter in .unprepare_pgtable()

The size parameter in .unprepare_pgtable() and arm_smmu_assign_table()
needs to be the same since the functions are complimentary.

Use PAGE_SIZE in both functions instead of relying on the calling
function for .unprepare_pgtable().

Change-Id: Ic6fade307360254329968e1b4548732d045b8205
Signed-off-by: Neeti Desai <neetid@codeaurora.org>