xen/iommu: smmu: Silence clang in arm_smmu_device_dt_probe()

xen/iommu: smmu: Silence clang in arm_smmu_device_dt_probe()

Clang 11 will throw the following error:

smmu.c:2284:18: error: cast to smaller integer type 'enum arm_smmu_arch_version' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]
        smmu->version = (enum arm_smmu_arch_version)of_id->data;
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The error can be prevented by initially casting to (uintptr_t) and then
enum.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>