iommu/arm-smmu: Be explicit about security mechanism

Up until now, the arm-smmu driver has only supported one type of
security mechanism: master-side access control.  However, in the near
future it will be getting support for slave-side access control, at
which point saying a domain is "secure" will be ambiguous.  Make the
distinction explicit by renaming arm_smmu_is_domain_secure to
arm_smmu_is_master_side_secure.

CRs-Fixed: 959535
Change-Id: Ie9bc077fe60d0b97c744fdb5b3f553cc056df27f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>