s390/sclp: Move early code from sclp_cmd.c to sclp_early.c

The early SCLP driver code in sclp_cmd.c belongs to sclp_early.c
because it is independent from the 'normal' SCLP driver. So move
it to sclp_early.c

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>