arm64: Force device mappings to be non-secure shared

Device mappings (currently) need to be emulated by the VMM so must be
mapped shared with the host.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
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Changes from v3:
 * Make use of pgprot_decrypted() in the definition of pgprot_device()