arm64: Document Arm Confidential Compute

Add some documentation on Arm CCA and the requirements for running Linux
as a Realm guest. Also update booting.rst to describe the requirement
for RIPAS RAM.

Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20241017131434.40935-12-steven.price@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>