ArmVirtPkg: RMM 1.0-rel0 - Add RSI_ERROR_UNKNOWN return code

The RMM 1.0-rel0 specification introduced a new error code to
indicate that the attestation token generation failed for an
unknown or IMPDEF reason.

Therefore, define the new error code and update the code to
handle the error accordingly.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>