s390/ap: add debug possibility for AP messages

This patch introduces two dynamic debug hexdump
invocation possibilities to be able to a) dump an
AP message immediately before it goes into the
firmware queue and b) dump a fresh from the
firmware queue received AP message.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>