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/[e-mail]/T/#m27f781ab60acadfed8a9e9642f30d5414a5e2df3

Signed-off-by: Masahiro Yamada <[e-mail]>
Tested-by: Jiri Slaby <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/Makefile

···
 OBJCOPYFLAGS	:= -O binary
 
 libs-y		+= arch/s390/lib/
-drivers-y	+= drivers/s390/
 
 boot		:= arch/s390/boot
 syscalls	:= arch/s390/kernel/syscalls

drivers/Makefile

···
 obj-$(CONFIG_HTE)		+= hte/
 obj-$(CONFIG_DRM_ACCEL)		+= accel/
 obj-$(CONFIG_CDX_BUS)		+= cdx/
+
+obj-$(CONFIG_S390)		+= s390/