iommu: use u32 for IOMMU page table entries

IOMMU page table entries are always 32 bits long, so use "u32"
rather than "unsigned long" which is not correct on arm64.

Change-Id: Ia8eedd7b480fe6a5b874fd31ae919a8f1dab5b61
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>