s390/kvm: Move bitfields for dat tables

Move and improve the struct definitions for DAT tables from gaccess.c
to a new header.

Once in a separate header, the structs become available everywhere. One
possible usecase is to merge them in the s390 pte_t and p?d_t
definitions, which is left as an exercise for the reader.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Link: https://lore.kernel.org/r/20240703155900.103783-3-imbrenda@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>