s390/cio: add eadm subchannel driver

This driver allows usage of EADM subchannels. EADM subchannels
act as a communication vehicle for SCM increments.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>