s390/ap: Externalize AP bus specific bitmap reading function

Rename hex2bitmap() to ap_hex2bitmap() and export it for external
use. This function will be used by the implementation of the vfio-ap
ap_config sysfs attribute.

Signed-off-by: "Jason J. Herne" <jjherne@linux.ibm.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Link: https://lore.kernel.org/r/20240415152555.13152-2-jjherne@linux.ibm.com


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>