s390/sclp: move early printk code to drivers

Move the early sclp printk code to the drivers folder where also the
rest of the sclp code can be found. This way it is possible to use the
sclp private header files for further cleanups.

Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>