s390/ipl: add support to control memory clearing for nvme re-IPL

Re-IPL for nvme is currently done by using diag 308 with the "Load Clear"
subcode, which means that all memory will be cleared.
This can increase re-IPL duration considerably on very large machines.

For list-directed IPL like nvme or fcp IPL, a "Load Normal" subcode was
introduced with z14. The "Load Normal" diag 308 subcode allows to re-IPL
without clearing memory.

This patch adds a new "clear" sysfs attribute to /sys/firmware/reipl/nvme,
which can be set to either "0" or "1" to disable or enable re-IPL with
memory clearing. The default value is "0", which disables memory clearing.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>