s390/sclp: Add support for Store Data SCLP interface

Add functions to retrieve data associated with an SCLP Store Data
entity. Automatically retrieve data for the "config" entity during
boot and make that data available to user-space via sysfs:

    /sys/firmware/sclp_sd/config/data
        Reading from this file will return config data contents.

    /sys/firmware/sclp_sd/config/reload
        Writing to this file will cause the latest version of data
        related to the config entity to be read from the SCLP interface.

Generate a KOBJ_CHANGE whenever new data is retrieved.

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