s390/ap: introduce new AP bus sysfs attribute features

Introduce a new AP bus sysfs attribute /sys/bus/ap/features
which shows the features from the QCI information.
Currently these feature bits are evaluated:

- QCI S bit is shown as 'APSC'
- QCI N bit is shown as 'APXA'
- QCI C bit is shown as 'QACT'
- QCI R bit is shown as 'RC8A'
- QCI B bit is shown as 'APSB'

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>