refactor(lib/attestation): replace panic() with RSI_ERROR_UNKNOWN

Instead of leading the system to panic, the RSI calls now exit
gracefully to the realms, indicating RSI_ERROR_UNKWNOWN in the
return code. This way RSI calls do not halt the system completely
and allow for the realms to recover from failure after issuing
RSI calls.

Change-Id: I689e05b9a888d89d7860ff0f1e7633ffa477d1c9
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>