s390/mem_detect: add get_mem_detect_online_total()

Add a function to get online memory in total. It is supposed to be used
in the decompressor as well as during early kernel startup.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>