s390: use obj-y to descend into drivers/s390/

The single build rule does not work with the drivers-y syntax. [1]

Use the standard obj-y syntax. It moves the objects from drivers/s390/
to slightly lower address, but fixes the reported issue.

[1]: https://lore.kernel.org/linux-kbuild/d57ba55f-20a3-b836-783d-b49c8a161b6e@kernel.org/T/#m27f781ab60acadfed8a9e9642f30d5414a5e2df3



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230721171358.3612099-1-masahiroy@kernel.org


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>