coco: guest: arm64: Add sysfs ABI for CCA Realm guests

Userspace has used the arm-cca-dev platform device to detect Arm CCA
Realm guests. Removing it without a replacement would break that
detection and would also leave userspace depending on kernel
device-model details.

Add /sys/firmware/cca/realm_guest as a stable ABI for detecting whether
the kernel is running as an Arm CCA Realm guest. Create the file only in
Realm world, where it reads 1.

Document the new ABI in Documentation/ABI/testing/sysfs-firmware-cca.

Link: https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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