iommu/arm-smmu: Fix a Null pointer dereference

Introduced by 99afd053.
"iommu/arm-smmu: Be explicit about security mechanism"

Internal error: Oops: 96000006 [#1] PREEMPT SMP
[<ffffffc0009f37d0>] arm_smmu_is_master_side_secure+0x14/0x28
[<ffffffc0009f029c>] io_pgtable_free_pages_exact+0x30/0x94
[<ffffffc0009f0388>] __arm_lpae_free_pgtable+0x88/0xf0
[<ffffffc0009f03e4>] __arm_lpae_free_pgtable+0xe4/0xf0
[<ffffffc0009f0410>] arm_lpae_free_pgtable+0x20/0x38
[<ffffffc0009f01cc>] free_io_pgtable_ops+0x24/0x30
[<ffffffc0009f4ce8>] arm_smmu_domain_destroy+0x1c/0xa8
[<ffffffc0009ed0cc>] iommu_domain_free+0x1c/0x34
[<ffffffc0009f35c4>] iommu_debug_device_profiling+0x660/0x6a4
[<ffffffc0009f361c>] iommu_debug_secure_profiling_show+0x14/0x24
[<ffffffc0001c4a2c>] seq_read+0x180/0x3b0

Found when running:
cat /sys/kernel/debug/iommu/tests/client_name/secure_profiling

Add a new smmu_domain variable to track whether the domain belongs to a
master-side or slave-side secure context.

Change-Id: Ib32cc6fb03f863522de10e416007114c6e91776f
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>