arm64: realm: Map unprotected address at S2 SMMU

For Realm, the non-secure memory is always at the top half of the IPA address
space. However, since the memory is aliased, we must always make sure the S2-SMMU
mappings are created to the "upper" aliased half.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>