iommu: free io pgtable during domain detach.

Pagetable pgd is allocated in arm_smmu_init_domain_context and is freed in
arm_smmu_domain_free. Client can attach/detach domain without freeing the
same. This causes memory leak. This was introduced with
commit fe676929 ("iommu/arm-smmu: fix a DOMAIN_ATTR_DYNAMIC memory
leak")

Free io pgtables in arm_smmu_destroy_domain_context, which is invoked at
the time of domain detach.

Change-Id: Ib53e4efdd8dca33437596f3f0ccaa8ccaae79ca0
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>