s390/ipl: add eckd support

This adds support to IPL from ECKD DASDs to linux.
It introduces a few sysfs files in /sys/firmware/reipl/eckd:

bootprog: the boot program selector
clear:    whether to issue a diag308 LOAD_NORMAL or LOAD_CLEAR
device:   the device to ipl from
br_chr:   Cylinder/Head/Record number to read the bootrecord from.
          Might be '0' or 'auto' if it should be read from the
	  volume label.
scpdata:  data to be passed to the ipl'd program.

The new ipl type is called 'eckd'.

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>