arm64: mm: Add top-level dispatcher for internal mem_encrypt API

Implementing the internal mem_encrypt API for arm64 depends entirely on
the Confidential Computing environment in which the kernel is running.

Introduce a simple dispatcher so that backend hooks can be registered
depending upon the environment in which the kernel finds itself.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240830130150.8568-4-will@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>