s390/ipl: add eckd dump support

This adds support to use ECKD disks as dump device
to linux. The new dump type is called 'eckd_dump', parameters
are the same as for eckd ipl.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>