s390/topology: Add sysctl handler for polarization

s390/topology: Add sysctl handler for polarization

Provide an additional path to set the polarization of the system, such
that a user no longer relies on the sysfs interface only and is able
configure the polarization for every reboot via sysctl control files.

The new sysctl can be set as follows:
 - s390.polarization=0 for horizontal polarization
 - s390.polarization=1 for vertical polarization

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>