arm64: rsi: Interfaces to query attestation token

Add interfaces to query the attestation token using
the RSI calls.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>