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>
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Changes since v6:
 * Mention "Realm Services Interface (RSI)" by name.
 * Add a brief explanation of the example earlycon line.