efi: arm64: Map Device with Prot Shared

Device mappings need to be emualted 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 since v4:
 * Reworked to use arm64_is_iomem_private() to decide whether the memory
   needs to be decrypted or not.