iommu/omap: Fix pointer cast -Wpointer-to-int-cast warnings on 64 bit

pointers should be casted to unsigned long to avoid
-Wpointer-to-int-cast warnings when compiling on 64-bit platform (e.g.
with COMPILE_TEST):

    drivers/iommu/omap-iommu.c: In function ‘omap2_iommu_enable’:
    drivers/iommu/omap-iommu.c:170:25: warning:
        cast from pointer to integer of different size [-Wpointer-to-int-cast]
      if (!obj->iopgd || !IS_ALIGNED((u32)obj->iopgd,  SZ_16K))
                             ^

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>