arm64: Enable memory encrypt for Realms

Use the memory encryption APIs to trigger a RSI call to request a
transition between protected memory and shared memory (or vice versa)
and updating the kernel's linear map of modified pages to flip the top
bit of the IPA. This requires that block mappings are not used in the
direct map for realm guests.

Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>