s390/ipl: add DEFINE_GENERIC_LOADPARM()

In the current code each reipl type implements its own pair of loadparm
show/store functions. Add a macro to deduplicate the code a bit.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Fixes: 87fd22e0 ("s390/ipl: add eckd support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>