lib: s390x: sie: Properly populate SCA

CPU0 is the only cpu that's being used but we should still mark it as
online and set the SDA in the SCA.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20221021063902.10878-7-frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>